Link to documentation in the readme

[JonRowe]

:wave: Thanks for all the hard work in this project, I was getting a bit frustrated trying to find any documentation for how ignore works (I want to ignore some directory globs that don't seem to be working from .ignore) and it took a search through the issues to discover the wiki, as almost every Github project has a wiki tab (its the default) with nothing in it.

To help those people who are a bit scatter brained when it comes to looking for docs and expect there to be a "docs" link somewhere on the webpage or readme, I've added one, suggested improvements obviously welcome :)