Document prominently the existence of a file_finders plugin

[apljungquist]

I discovered this fact when troubleshooting files that should not have been included according to MANIFEST.in were being included.

Factoring out the feature to a separate package distribution was suggested in #190 but was rejected. Though I would have preferred a way to disable the feature, simply documenting it prominently would have saved me a lot of time.

A similar fix was attempted in #53.

The problem is somewhat aggravated by an issue with setuptools; When, in the course of figuring out why so many files where being included, I removed setuptools_scm the files were still being included.

[webknjaz]

Ref: #190 (comment)