warn about server side hooks

examples/pre-receive.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+while read oldrev newrev refname
+do
+    branch=$(git rev-parse --symbolic --abbrev-ref $refname)
+    if [ "master" = "$branch" ]; then
+        echo "wrong branch"
+        exit 1
+    fi
+done