Update developer instructions for cloning the Gerrit project

The core plugins are now included as git submodules.

When cloning the gerrit project, the --recursive option must be
used otherwise the submodules will not be cloned and the build
will not work.

Update the documentation accordingly.

Change-Id: I02decd68a1de061b60d3e7b6ce2ffdd24d12505a
(cherry picked from commit 4f2be538e7fa9f5ab67edf963615bd721a027285)
1 file changed