Add a paragraph on documentation to CONTRIBUTING

Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>

CONTRIBUTING

@@ -69,3 +69,7 @@ the acceptance and review process faster:
     5.  When at all possible, patches should include tests. These can either be
     added to an existing test, or completely new.  Please see test/README
     for information on the test framework.
+
+    6.  New features or changed functionality must include documentation. Please
+    look at the "pod" files in doc/apps, doc/crypto and doc/ssl for examples of
+    our style.