Added explanation for "contains banned commit" error

Added a documentation page that explains the
"contains banned commit ..." error in details.

Change-Id: Ie574a0cf84be5d97d11c2eaee3cabbcf759c3ce3
Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>

Documentation/error-contains-banned-commit.txt

@@ -0,0 +1,21 @@
+contains banned commit ...
+==========================
+
+With this error message Gerrit rejects to push a commit that is
+banned or that would merge in an ancestor that is banned.
+
+If a commit was identified as a bad commit (e.g. because it contains
+coding that violates intellectual property) and because of this it
+was removed from the central git repository it can be marked as
+banned. Gerrit will then prevent that this commit ever enters the
+repository again by rejecting every push of such a commit with the
+error message "contains banned commit ...".
+
+If you have commits that you want to push that are based on a banned
+commit you may want to link:http://www.kernel.org/pub/software/scm/git/docs/git-cherry-pick.html[cherry-pick] them onto a clean base and push
+them again.
+
+
+GERRIT
+------
+Part of link:error-messages.html[Gerrit Error Messages]

Documentation/error-messages.txt

@@ -13,6 +13,7 @@ Error Messages
 * link:error-change-closed.html[change ... closed]
 * link:error-change-does-not-belong-to-project.html[change ... does not belong to project ...]
 * link:error-change-not-found.html[change ... not found]
+* link:error-contains-banned-commit[contains banned commit ...]
 * link:error-has-duplicates.html[... has duplicates]
 * link:error-invalid-changeid.html[invalid Change-Id line format in commit message]
 * link:error-missing-changeid.html[missing Change-Id in commit message]