misc: Update CONTRIBUTING with gerrit changes

Change-Id: I39b5a784a056195a5e71fd21b3abb5fc91ab70e7
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/11872
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3cfad9f..92d8ea8 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -244,13 +244,21 @@
  git commit --amend
 ```
 
-Push change to gerrit as a draft
---------------------------------
+Push change to gerrit as a draft/private
+----------------------------------------
+
+See https://gerrit-review.googlesource.com/Documentation/intro-user.html#private-changes
+for details on private gerrit changes.
 
 ```
- git push origin HEAD:refs/drafts/master
+ git push origin HEAD:refs/for/master%private
 ```
 
+Once you have pushed your change as "private", you can log onto [gerrit]
+(https://gem5-review.googlesource.com) and once you're happy with the commit
+click the "unmark private" which may be hidden in the "more options" dropdown
+in the upper right corner.
+
 Push change bypassing gerrit
 -----------------------------