Docutils FAQ already recommends .txt extension

[benoitbryon]

See

documentation-style-guide-sphinx/docs/style-guide.txt

Line 24 in d167799

Suffix filenames with the ``.txt`` extension.

and http://docutils.sourceforge.net/FAQ.html#what-s-the-standard-filename-extension-for-a-restructuredtext-file

=> The standard is .txt. This convention is on docutils website!
=> Don't use Sphinx's default which is .rst!