Outdated Documentation: Async Support

[kevdog824]

Disclaimer: I have searched the project's open issues here on GitHub and did not find any related issues

Documentation provided for filelock library readthedocs page is outdated. Currently, the documentation states:

This library currently does not support asyncio. We’d recommend adding an asyncio variant though if someone can make a pull request for it, see here.

The "see here" link links to #99.

However, I can see that asyncio support for the library was added in version 3.15.0 in #332. #332 includes only a single usage example but does not have extensive documentation as far as I can tell. Please update the documentation to include usage instructions and examples for async use.

If no one has capacity to help with this, I might be able to contribute at some later point.

Thanks!