Add README to documentation Index if no other docs

[theory]

Quite a few distributions have only one extension and no documentation file other than the README. Authors can get around this by adding the README under the docfile key in the provides section, but it seems silly.

So, add a rule to the indexer such that, if the distribution:

1. Has only one extension
2. Has no documentation file for that extension
3. Has a README

Then automatically index the README as documentation.

[theory]

It looks as though if authors set the docfile key to the README it doesn't show up at all. Example: mongo_fdw 1.0.0. So authors can't work around it. :-(