Add a note on CHANGES and NEWS in CONTRIBUTING

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from openssl#6249)

CONTRIBUTING

@@ -53,3 +53,20 @@ guidelines:
     documentation. Please look at the "pod" files in doc/man[1357] for
     examples of our style. Run "make doc-nits" to make sure that your
     documentation changes are clean.
+
+    7.  For user visible changes (API changes, behaviour changes, ...),
+    consider adding a note in CHANGES.  This could be a summarising
+    description of the change, and could explain the grander details.
+    Have a look through existing entries for inspiration. 
+    Please note that this is NOT simply a copy of git-log oneliners.
+    Also note that security fixes get an entry in CHANGES.
+    This file helps users get more in depth information of what comes
+    with a specific release without having to sift through the higher
+    noise ratio in git-log.
+
+    8.  For larger or more important user visible changes, as well as
+    security fixes, please add a line in NEWS.  On exception, it might be
+    worth adding a multi-line entry (such as the entry that announces all
+    the types that became opaque with OpenSSL 1.1.0).
+    This file helps users get a very quick summary of what comes with a
+    specific release, to see if an upgrade is worth the effort.