add help page for web hooks management using rake tasks

doc/raketasks/web_hooks.md

+### Add a web hook for **ALL** projects:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:add URL="http://example.com/hook"
+
+
+### Add a web hook for projects in a given **NAMESPACE**:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:add URL="http://example.com/hook" NAMESPACE=acme
+
+
+### Remove a web hook from **ALL** projects using:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:rm URL="http://example.com/hook"
+
+
+### Remove a web hook from projects in a given **NAMESPACE**:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:rm URL="http://example.com/hook" NAMESPACE=acme
+
+
+### List **ALL** web hooks:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:list
+
+
+### List the web hooks from projects in a given **NAMESPACE**:
+
+    RAILS_ENV=production bundle exec rake gitlab:web_hook:list NAMESPACE=/
+
+> Note: `/` is the global namespace.
+