Extend explanation of 'prohibited by Gerrit' error

New users often run into the 'prohibited by Gerrit'
error because they accidentally try to bypass code
review (e.g. in the push specification they
configure 'refs/heads/master' instead of
'refs/for/master' as destination ref). Since this
happens quite often this case is now explained in
an own paragraph in the 'prohibited by Gerrit'
documentation.

Change-Id: Icda2eccf7f8febbf48680b2d492a816641f1f357
Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>

Documentation/error-prohibited-by-gerrit.txt

diff --git a/Documentation/error-prohibited-by-gerrit.txt b/Documentation/error-prohibited-by-gerrit.txt
index 9aaef86..d8a9dd3 100644
--- a/Documentation/error-prohibited-by-gerrit.txt
+++ b/Documentation/error-prohibited-by-gerrit.txt
@@ -14,6 +14,14 @@
 3. if you push a tag without sufficient privileges in the link:access-control.html#category_pTAG['Push Tag']
    category
 
+For new users it happens often that they accidentally try to bypass
+code review. The push then fails with the error message 'prohibited
+by Gerrit' because the project didn't allow to bypass code review.
+Bypassing the code review is done by pushing directly to refs/heads/*
+(e.g. refs/heads/master) instead of pushing to refs/for/* (e.g.
+refs/for/master). Details about how to push commits for code review
+are explained link:user-upload.html#push_create[here].
+
 
 GERRIT
 ------