Add document for steps before release.

[dhruvdcoder]

Add a RELEASE.md describing steps that need to be taken before making a release.

1. Update the version in setup.py
2. Check if the test coverage is above a threshold.
3. Check if the CHANGELOG is up-to-date.
4. Create a tag matching the version in the setup.py. Copy the "unreleased" section of the CHANGELOG into the description for the tag and release.