Added doc about release pace

I think this closes #146

[makedoc]

user-doc/Intro.txt

@@ -80,7 +80,25 @@ or enhanced sampling calculations.
 \page Changelog Change Log
 
 Here you can find a history of changes across different PLUMED versions.
-We mostly add new features without breaking exising ones.
+The future releases are expected to follow more or less the pace
+of the old release. This means:
+- Approximately once per year, after summer, a new release (2.X). These releases
+  typically group together all the features that were contributed during the
+  year.
+- Approximately every three month, we announce a patch (e.g. 2.2.X).
+  This typically contains bug fixes, and could occasionally contain a new feature.
+
+A few months before each new release we provide a beta release.
+We typically maintain release branches until the fifth patch release (2.X.5),
+which should come out approximately 15 month after the original release (2.X).
+After that, branches are not supported anymore.
+
+Notice that occasionally we publish patches on the mailing list.
+These patches are always included in the following release, but we encourage
+users that want to be up to date to follow the mailing list.
+
+Below you can find change logs for all the published releases.
+We mostly add new features without breaking existing ones.
 However, some of the changes lead to incompatible behavior.
 In the Change Log we try to give as much visibility as possible to these changes
 to avoid surprises.