CONTRIBUTING: Mention design document template

Add a reference to the design template and suggest its use in the
Planning Changes section of CONTRIBUTING.md.

The design template includes discussion points both on the design,
alternatives, and impacts to the rest of the project and the review
gives a place for others to comment.

Change-Id: Ie9bdbca32ef1e732dbf2fa9bcdccb5121adfa149
Signed-off-by: Milton Miller <miltonm@us.ibm.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 62cbda5..7fae17f 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -146,6 +146,10 @@
 via the OpenBMC Discord server or email list to start
 the discussion.  You may find a better way to do what you need.
 
+If the feedback seems favorable or requests more details, continue
+by submitting the design to gerrit starting with a copy of the
+[design_template](https://github.com/openbmc/docs/blob/master/design/design_template.md).
+
 Submitting changes
 ------------------