Add a basic README file (fixes #3)

Here's a basic README file which we can improve in future branches eg
 * Add installation instructions (#4)
 * Add testing instructions (#6)
 * Add a travis-ci badge (#5)

Fixes: #3
:question: Other projects here have a new line after the heading (see https://raw.githubusercontent.com/twisted/twistedchecker/master/README.rst). I'm not sure if this is a requirement but it looks nicer to me.