Update release procedure

Author: stefanv

RELEASE.txt

@@ -1,11 +1,13 @@
 How to make a new release of ``cesium``
 =======================================
 
-- Update the version number in ``cesium/version.py`` and commit
+- Add the version number as an annotated tag in git::
 
-- Add the version number as a tag in git::
+   git tag -s -m "Description of release" v0.X.Y
 
-   git tag -m "Description of release" v0.X.Y
+  You can look at existing tags with::
+
+   git tag -l -n9
 
 - Push the new meta-data to github::