doc: link to the release cycle from contribute.html

Change-Id: Ia5d41b66006682084fcbfac3da020946ea3dd116
Reviewed-on: https://go-review.googlesource.com/13093Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/contribute.html

@@ -40,6 +40,14 @@ It also checks that the design is sound before code is written;
 the code review tool is not the place for high-level discussions.
 </p>
 
+<p>
+When planning work, please note that the Go project follows a
+<a href="https://golang.org/wiki/Go-Release-Cycle">six-month
+development cycle</a>. The latter half of each cycle is a three-month
+feature freeze during which only bug fixes and doc updates are accepted.
+New work cannot be submitted during a feature freeze.
+</p>
+
 <h2 id="Testing">Testing redux</h2>
 
 <p>