Add LICENSE.txt and CONTRIBUTING

[codehearts]

This project should have a copy of the GNU GPLv3 included with it. Also update the copyright information in the readme, and add a file for contributing.

[robohobbit]

Could some documentation be added? E.g. What's the gcalertrc syntax?
Even just adding a sample or skel gcalertrc when installing would be helpful. Thanks!

[codehearts]

The gcalertrc syntax is documented in the script's usage output, but it'd be great to have it in the readme. I'm planning on changing the syntax of it since it's just a list of command line args, one per line