Add project documentation via README file

This is a quick draft of a simple README file for the mdtest repo. It
includes the following:

- A description of the project, quoted from the source files

- Instructions on how to run the tests

- A sample shell script (bash) that details a simple way of running the
  tests against the lib branch of php-markdown (creating a wrapper
  script as well as a Makefile)

- The license, quoted from the source files

This increased documentation helps to lower the barrier to entry for
potential contributors, and also makes running the test suite easy.