Make the precommit hook explain some environmental setup steps

[Aaron Wells]

While setting up a lot of QA users for Mahara, we discovered that it's easy to miss steps in setting up your Mahara dev environment, between setting things up for Gerrit, Git, and our own custom make script.

I think it would be smart to add some logic to the make script, that checks for common mistakes and tells you how to fix them. Like, checking for the commit-msg script, the presence of the libxml2-utils package, and maybe adding links to the developer environment setup instructions in the wiki.