Fix git commit operation to gerrit

1. Add the `-a` option for adding changes automatically. It is
reported several missing-adding mistakes in past submissions.
2. Add notice on inputing useful commit msg.

Change-Id: If3d13d27f1df139616e95c1e3b0d6292d33efb2c
Signed-off-by: Baohua Yang <baohyang@cn.ibm.com>

docs/Gerrit/gerrit.md

@@ -75,9 +75,11 @@ git checkout -b issue-nnnn
 please read [this document](changes.md).
 
 ```
-git commit -s
+git commit -s -a
 ```
 
+Then input precise and readable commit msg and submit.
+
 6. Any code changes that affect documentation should be accompanied by
 corresponding changes (or additions) to the documentation and tests. This
 will ensure that if the merged PR is reversed, all traces of the change will