Update to explain delays.

The only use of the master branch in this repo is for these messages, so
updates are very rare. This can lead to confusion when folks first load
the github webview.

README.md

@@ -2,7 +2,12 @@ gecko-projects
 ==============
 
 This repository is synced from hg.mozilla.org and should be read-only.
-It is intended to be used with gecko-dev.  For instance, to add the 'alder' branch to your local repository:
+
+Note: 'master' branch is rairly updated - view the branch you care about
+to see how recent it is. It should be within ~30 minutes of
+hg.mozilla.org copy.
+
+This repo is intended to be used with gecko-dev.  For instance, to add the 'alder' branch to your local repository:
 
     git clone https://github.com/mozilla/gecko-dev.git
     cd gecko-dev