add bullet point on modifying an existing PR

CONTRIBUTING.md

 # How to contribute to transformers?
 
 Everyone is welcome to contribute, and we value everybody's contribution. Code
-is thus not the only way to contribute. Answering questions, helping others,
-reaching out and improving the documentations are immensely valuable to the
-community.
+is thus not the only way to help the community. Answering questions, helping
+others, reaching out and improving the documentations are immensely valuable to
+the community.
 
 It also helps us if you spread the word: reference the library from blog posts
-on the awesome projects it made possible, shout out on twitter every time it has
+on the awesome projects it made possible, shout out on Twitter every time it has
 helped you, or simply star the repo to say "thank you".
 
 ## You can contribute in so many ways!
@@ -14,7 +14,7 @@ helped you, or simply star the repo to say "thank you".
 There are 4 ways you can contribute to transformers:
 * Fixing outstanding issues with the existing code;
 * Implementing new models;
-* Contributing to the examples, or to the documentation;
+* Contributing to the examples or to the documentation;
 * Submitting issues related to bugs or desired new features.
 
 *All are equally valuable to the community.*
@@ -28,18 +28,17 @@ feedback.
 ### Did you find a bug?
 
 The transformers are robust and reliable thanks to the users who notify us of
-the problems they encounter.
+the problems they encounter. So thank you for reporting an issue.
 
-So thank you for reporting an issue. First, we would really appreciate it if you
-could **make sure the bug was not already reported** (use the search bar on
-Github under Issues).
+First, we would really appreciate it if you could **make sure the bug was not
+already reported** (use the search bar on Github under Issues).
 
 Did not find it? :( So we can act quickly on it, please follow these steps:
 
 * Include your **OS type and version**, the versions of **Python**, **PyTorch** and
   **Tensorflow** when applicable;
 * A short, self-contained, code snippet that allows us to reproduce the bug in
-  less than 30s.
+  less than 30s;
 * Provide the *full* traceback if an exception is raised.
 
 To get the OS and software versions, execute the following code and copy-paste
@@ -54,13 +53,14 @@ import tensorflow; print("Tensorflow", tensorflow.__version__)
 
 ### Do you want to implement a new model?
 
-Please provide the following:
+Awesome! Please provide the following information:
 
-* Short description of the model and link to the paper
-* Link to the implementation if open-source
-* Link to the model weights if they are available
+* Short description of the model and link to the paper;
+* Link to the implementation if it is open-source;
+* Link to the model weights if they are available.
 
-Let us know if you are willing to contribute so we can best guide you. 
+If you are willing to contribute the model yourself, let us know so we can best
+guide you.
 
 ### Do you want a new feature (that is not a model)?
 
@@ -73,19 +73,19 @@ A world-class feature request addresses the following points:
     about it!
   * Is it something you worked on and think could benefit the community?
     Awesome! Tell us what problem it solved for you.
-2. Write a *full paragraph* describing the feature.  
-3. Provide a **code snippet** that demonstrates its future use.
-4. In case this is related to a paper, please provide a link
+2. Write a *full paragraph* describing the feature;
+3. Provide a **code snippet** that demonstrates its future use;
+4. In case this is related to a paper, please attach a link;
 5. Attach any additional information (drawings, screenshots, etc.) you think may help.
 
-If your issue is well-written we're already 80% of the way there by the time you
+If your issue is well written we're already 80% of the way there by the time you
 post it.
 
 ## Start contributing! (Pull Requests)
 
 Before writing code, we strongly advise you to search through the exising PRs or
-issues to make sure that nobody is already working on the same thing. It is
-always a good idea to open an issue to get some feedback.
+issues to make sure that nobody is already working on the same thing. If you are
+unsure, it is always a good idea to open an issue to get some feedback.
 
 You will need basic `git` proficiency to be able to contribute to
 `transformers`. `git` is not the easiest tool to use but it has the greatest
@@ -95,7 +95,7 @@ Git](https://git-scm.com/book/en/v2) is a very good reference.
 Follow these steps to start contributing:
 
 1. Fork the [repository](https://github.com/huggingface/transformers) by
-   clicking on the 'Fork' button. This creates a copy of the code
+   clicking on the 'Fork' button on the repository's page. This creates a copy of the code
    under your github user account.
 2. Clone your fork to your local disk, and add the base repository as a remote:
    
@@ -113,7 +113,7 @@ Follow these steps to start contributing:
    
    **do not** work on the `master` branch.
    
-4. Set up a development environment by running in a virtual environment:
+4. Set up a development environment by running the following command in a virtual environment:
 
    ```bash
    $ pip install -r requirements-dev.txt
@@ -146,13 +146,19 @@ Follow these steps to start contributing:
 6. Once you are satisfied (**and the checklist below is happy too**), go to the
    webpage of your fork on Github. Click on 'Pull request' to send your changes
    to the project maintainers for review.
+   
+7. It's ok if maintainers ask you for changes. It happens to core contributors
+   too! So everyone can see the changes in the Pull request, work in your local
+   branch and push the changes to your fork. They will automatically appear in
+   the pull request.
 
 
 ### Checklist
 
 1. The title of your pull request should be a summary of its contribution;
 2. If your pull request adresses an issue, please mention the issue number in
-   the pull request description to make sure they are linked;
+   the pull request description to make sure they are linked (and people
+   consulting the issue know you are working on it);
 3. To indicate a work in progress please prefix the title with `[WIP]`. These
    are useful to avoid duplicated work, and to differentiate it from PRs ready
    to be merged;