Extract linter documentation into separate document

We now have enough linters that it's not going to be practical to
document them in the main README.

Create a separate document and link to it from the main README. In the
process, add additional justification for some of the linters.

Change-Id: I5a2eda7c06ec51a78fa3524a468da2f6bdc8bcc6
Reviewed-on: http://gerrit.causes.com/42340
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>