update installation guide (#1701)

docs/INSTALL.md

@@ -42,8 +42,8 @@ cd mmdetection
 d. Install mmdetection (other dependencies will be installed automatically).
 
 ```shell
-python setup.py develop
-# or "pip install -v -e ."
+pip install mmcv
+python setup.py develop  # or "pip install -v -e ."
 ```
 
 Note:
@@ -53,6 +53,9 @@ It is recommended that you run step d each time you pull some updates from githu
 
 2. Following the above instructions, mmdetection is installed on `dev` mode, any local modifications made to the code will take effect without the need to reinstall it (unless you submit some commits and want to update the version number).
 
+3. If you would like to use `opencv-python-headless` instead of `opencv-python`,
+you can install it before installing MMCV.
+
 ### Another option: Docker Image
 
 We provide a [Dockerfile](../docker/Dockerfile) to build an image.