Add MU plugin description to readme

[A5hleyRich]

Didn't expect it as the wp.org description's "Features" didn't say an MU plugin would be added for it, would be cool of that was clarified to save queries from numpties like me!

[Jamesclark32]

@A5hleyRich Do you happen to remember more context for this?

What needs to change in the description?

[A5hleyRich]

It's been a while, but my guess is it's about adding a mention of the MU plugin that's installed when our WP plugin is activated. The MU plugin is responsible for setting the correct error log location.