+You can check out a specific external PACKAGE into the `packages'
+directory with these commands:
+
+ cd packages
+ git clone --reference .. --single-branch --branch externals/PACKAGE $(git config remote.origin.url) PACKAGE
+
+If you already have a packages/PACKAGE directory with a previous
+checkout, you can update it like this:
+
+ cd packages/PACKAGE
+ git pull
+