update README (#637)

--user will mislead some users to install NNI to their home directory while they are using venv or whatever

It should be added only when someone wants to install to home directory. For example, if all users shared a global python and one has no privileges

README.md

@@ -97,12 +97,14 @@ We encourage researchers and students leverage these projects to accelerate the
 
 **Install through pip** 	
 * We support Linux and MacOS in current stage, Ubuntu 16.04 or higher, along with MacOS 10.14.1 are tested and supported. Simply run the following `pip install` in an environment that has `python >= 3.5`.	
+
 ```bash
     python3 -m pip install --upgrade nni
 ```
-* Note:
-  * If you are in docker container (as root), please remove `--user` from the installation command.
-  * If there is any error like `Segmentation fault`, please refer to [FAQ](docs/FAQ.md)
+Note:
+
+* `--user` can be added if you want to install NNI in your home directory, which does not require any special privileges.
+* If there is any error like `Segmentation fault`, please refer to [FAQ](docs/FAQ.md)
 
 **Install through source code**
 * We support Linux (Ubuntu 16.04 or higher), MacOS (10.14.1) in our current stage.