Added base for UnitTests and updated README.md to reflect this

Here I've setup the basics for UnitTests.

I've added the vendor directory to .gitignore based on [this advice](https://getcomposer.org/doc/faqs/should-i-commit-the-dependencies-in-my-vendor-directory.md) and also realised that PHPUnit needs to be installed globally and not as part of the package so I've not added it to the composer.json file (which I've not actually changed).

I added instructions for contributors to run the tests at the bottom of README.md and modified the Installation heading to reflect the new requirement of composer (now needed if you want to run the tests).