Added explanation for "Not a Gerrit administrator" error

Added a documentation page that explains the
"Not a Gerrit administrator" error.

Change-Id: Iedf7164ee7079116e7b7934ac785baf0281f615a
Signed-off-by: Edwin Kempin <edwin.kempin@gmail.com>
diff --git a/Documentation/error-messages.txt b/Documentation/error-messages.txt
index 2f0a8f2..eb3c993 100644
--- a/Documentation/error-messages.txt
+++ b/Documentation/error-messages.txt
@@ -15,6 +15,7 @@
 * link:error-missing-changeid.html[missing Change-Id in commit message]
 * link:error-multiple-changeid-lines.html[multiple Change-Id lines in commit message]
 * link:error-no-new-changes.html[no new changes]
+* link:error-not-a-gerrit-administrator[Not a Gerrit administrator]
 * link:error-not-a-gerrit-project.html[not a Gerrit project]
 * link:error-not-valid-ref.html[not valid ref]
 * link:error-permission-denied.html[Permission denied (publickey)]
diff --git a/Documentation/error-not-a-gerrit-administrator.txt b/Documentation/error-not-a-gerrit-administrator.txt
new file mode 100644
index 0000000..0468d83
--- /dev/null
+++ b/Documentation/error-not-a-gerrit-administrator.txt
@@ -0,0 +1,14 @@
+Not a Gerrit administrator
+==========================
+
+With this error message Gerrit rejects to execute a SSH command that
+requires administrator privileges if the user is not a Gerrit
+administrator.
+
+The Gerrit link:cmd-index.html#admin_commands[administrator commands] can only be executed by users who
+are member of the Gerrit link:access-control.html#administrators[Administrators] group.
+
+
+GERRIT
+------
+Part of link:error-messages.html[Gerrit Error Messages]