Move linter documentation into separate files

In order to make the README more navigable, move the linter
documentation into separate files (one for the main linters and another
for Compass linters).

This will make it easier to link to other documentation when new linters
are added.

Change-Id: Ic74ab5edd6d1fdbb2d1003eb0cc32c8e7d679726
Reviewed-on: https://gerrit.causes.com/33674
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>