how-to-release: add notes about Launchpad

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@64524 15284696-431f-4ddb-bdfa-cd5b030d7da7

doc/howto_release.txt

@@ -176,6 +176,12 @@ Contents
 
      copy_addon 700beta3 7.0.0beta3
 
+ o Launchpad notes:
+
+   Create milestone and release: https://launchpad.net/grass/+series
+
+   Upload tarball for created release
+
 2. Tell others about it:
 
  * If release candidate: