docs/releasing.md: short doc describing how to make a release.

Author: mhagger

docs/releasing.md

@@ -0,0 +1,20 @@
+# How to create a release of `git-when-merged`
+
+1.  Create a release tag:
+
+        VERSION=1.2.3
+        git tag -s -m "git-when-merged version $VERSION" v$VERSION
+
+2.  Create a test release (without uploading):
+
+        tox -e publish
+
+3.  Push the tag:
+
+        git push origin v$VERSION
+
+4.  Create and upload a release:
+
+        tox -e publish -- upload
+
+See [this PR](https://github.com/mhagger/git-when-merged/pull/22) for more information.