Add release instructions (#1162)

Add instructions on how to produce a Sceptre release.

Signed-off-by: Thanh Ha <zxiiro@gmail.com>

RELEASE.md

@@ -0,0 +1,12 @@
+# Sceptre release process
+
+1. Bump version in `./sceptre/__init__.py` & `./setup.cfg`
+2. Update Changelog with details from
+   `git log --no-merges --format='%<(20)%an' v1.0.0..HEAD | sort | uniq`
+   where the version number is the git tag of the last release.
+3. Create a PR for above changes
+4. Once PR is merged, `git pull` the changes to sync the *master* branch
+5. `git tag -as vX.Y.Z`
+6. `git push origin vX.Y.Z` (CI/CD publishes to PyPi)
+7. Announce release to the #sceptre channel on og-aws Slack with a link to
+   the latest changelog