RELEASE.md: clarify release process

pohly · pohly · commit 51e16be61535 · 2019-01-21T10:18:56.000+01:00
It's worth calling out explicitly that only the master branch is
maintained.
diff --git a/RELEASE.md b/RELEASE.md
@@ -1,3 +1,5 @@
 # Release Process
 
-TODO: describe the release process for this project
+No tagged releases are planned at this point. The intention is to keep
+the master branch in a state such that it can be used for all
+supported branches in downstream repos which use these files.
