Update CONTRIBUTING documentation

Changes:
    - Added reference link to the formatting commit message page

Tested:
    Not applicable, as this is a documentation change.

Change-Id: I9a130b8f9ab40d98302a8ac702c4a4d714378063
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 851513d..94725d3 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -8,6 +8,10 @@
 -   Reference [OpenBMC CLA](https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md#submitting-changes-via-gerrit-server-to-openbmc)
 -   Reference [OpenBMC docs](https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md#submitting-changes-via-gerrit-server)
 
+Commit Message Guidelines
+-------------------------
+-   Reference [Formatting Commit Messages](https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md#formatting-commit-messages)
+
 Links
 -----