elaborate on commit style

content/contribute.rst

@@ -159,4 +159,11 @@ Coding style
 * Write tests for all the important functionality you add.  Be sure not to
   break existing tests.
 
-* Create a logical sequence of commits with clear commit messages.
+* Create a logical sequence of commits with clear commit messages.  Each
+  commit message consists of a summary line, and, whenever necessary, a
+  more detailed explanation.  Both parts are separated by a blank line
+  and wrapped to 72 characters (unless quoting error messages and such).
+  The summary line should be as clear as possible.  The detailed
+  explanation should be included whenever it helps to motivate or
+  understand the commit.  Bear in mind that the merge request text and
+  discussions are not part of a commit.