Use README.md as index.html also if use_directory_urls is false

[oprypin]

Right now, just like any foo/index.md, any foo/README.md becomes foo/index.html, but that works only if use_directory_urls is on.
But the use_directory_urls feature is for changing foo/bar.md into foo/bar/index.html and should not prevent the former from working.

I'm hoping that this is more of an oversight (due to the source code for the two things being very related) than an intentional exclusion.
So I'm sending a small pull request to fix that and also unify the code paths. But feel free to discuss this as an issue in case it's not as agreeable as I thought.

[waylan]

I just searched through the old discussions about this and can't find any reason for the current behavior of only working when use_directory_urls is True. Additionally, the documentation makes no suggestion of a difference in behavior here. Seems like this was an oversight. Thanks for the report and the work.

[waylan]

One thing is missing. We need a note added to the release notes. Here:

mkdocs/docs/about/release-notes.md

Lines 24 to 33 in 3bada39

	## Version 1.1.1 (in development)
	* Bugfix: Ignore links which start with a backslash (#1680).
	* Bugfix: Pass `builder` to the `on_serve` event so that it can be passed to
	`server.watch` by plugins (#1952).
	* Bugfix: Use `lunr[languages]==0.5.8` to avoid `nltk` incompatibilities (#2062).
	* Bugfix: Ensure wheel is Python 3 only (#2021).
	* Bugfix: Clean up `dev_addr` validation and disallow `0.0.0.0` (#2022).
	* Add support for `min_search_length` parameter for search plugin (#2014).
	* Bugfix: `readthedocs` theme `code` colors (#2027).