[Issue & PR Template] Add Issue Template & PR Template & Contribution Guide (#192)

* add issue template

* add contribution guide and PR Template, adapted from mxnet

* fix typo

.github/ISSUE_TEMPLATE/bug-report.md

+---
+name: "\U0001F41B Bug Report"
+about: Submit a bug report to help us improve DGL
+
+---
+
+## 🐛 Bug
+
+<!-- A clear and concise description of what the bug is. -->
+
+## To Reproduce
+
+Steps to reproduce the behavior:
+
+1.
+1.
+1.
+
+<!-- If you have a code sample, error messages, stack traces, please provide it here as well -->
+
+## Expected behavior
+
+<!-- A clear and concise description of what you expected to happen. -->
+
+## Environment
+
+ - DGL Version (e.g., 1.0):
+ - Backend Library & Version (e.g., PyTorch 0.4.1):
+ - OS (e.g., Linux):
+ - How you installed DGL (`conda`, `pip`, source):
+ - Build command you used (if compiling from source):
+ - Python version:
+ - CUDA/cuDNN version:
+ - GPU models and configuration:
+ - Any other relevant information:
+
+## Additional context
+
+<!-- Add any other context about the problem here. -->
\ No newline at end of file

.github/ISSUE_TEMPLATE/documentation.md

+---
+name: "\U0001F4DA Documentation"
+about: Report an issue related to docs.dgl.ai
+
+---
+
+## 📚 Documentation
+
+<!-- Please specify whether it's tutorial part or API reference part-->
+<!-- A clear and concise description of what content in docs.dgl.ai is an issue.-->
\ No newline at end of file

.github/ISSUE_TEMPLATE/feature-request.md

+---
+name: "\U0001F680Feature Request"
+about: Submit a proposal/request for a new DGL feature
+
+---
+
+## 🚀 Feature
+<!-- A clear and concise description of the feature proposal -->
+
+## Motivation
+
+<!-- Please outline the motivation for the proposal. Is your feature request related to a problem? e.g., I'm always frustrated when [...]. If this is related to another GitHub issue, please link here too -->
+
+## Alternatives
+
+<!-- A clear and concise description of any alternative solutions or features you've considered, if any. -->
+
+## Pitch
+
+<!-- A clear and concise description of what you want to happen. -->
+
+## Additional context
+
+<!-- Add any other context or screenshots about the feature request here. -->
\ No newline at end of file

.github/ISSUE_TEMPLATE/questions-help-support.md

+---
+name: "❓Questions/Help/Support"
+about: Do you need support? We have resources.
+
+---
+
+## ❓ Questions and Help
+
+### Please note that this issue tracker is not a help forum and this issue will be closed.
+
+Our primary means of support is our discussion forum:
+
+- [Discussion Forum](http://discuss.dgl.ai/)
\ No newline at end of file

.github/PULL_REQUEST_TEMPLATE.md

+IMPORTANT: Please do not create a Pull Request without creating an issue first.
+
+Any change needs to be discussed before proceeding. Failure to do so may result in the rejection of the pull request.
+
+Please provide enough information so that others can review your pull request:
+
+## Description
+Explain the details for making this change. What existing problem does the pull request solve?
+
+## Test Plan
+Describe the test plan (required)
+
+## Checklist
+### Essentials
+Please feel free to remove inapplicable items for your PR.
+- [ ] The PR title starts with [$CATEGORY] (such as [Model], [Doc], [Feature]])
+- [ ] Changes are complete (i.e. I finished coding on this PR)
+- [ ] All changes have test coverage
+- [ ] Code is well-documented
+- [ ] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
+- [ ] Related issue is referred in this PR
+
+### Changes ###
+- [ ] Feature1, tests, (and when applicable, API doc)
+- [ ] Feature2, tests, (and when applicable, API doc)
+
+## Comments ##
+Add any other context about the pull request here.

CONTRIBUTING.md

+## Contributing to DGL
+
+If you are interested in contributing to DGL, your contributions will fall
+into two categories:
+1. You want to propose a new Feature and implement it
+    - post about your intended feature, and we shall discuss the design and
+    implementation. Once we agree that the plan looks good, go ahead and implement it.
+2. You want to implement a feature or bug-fix for an outstanding issue
+    - Look at the outstanding issues
+    - Especially look at the Low Priority and Medium Priority issues
+    - Pick an issue and comment on the task that you want to work on this feature
+    - If you need more context on a particular issue, please ask and we shall provide.
+
+Once you finish implementing a feature or bugfix, please send a Pull Request.
+