Add README description for g:matchup_mouse_enabled

[bew]

Hello!

I just installed this plugin and read the whole README to see what I can do, and since a lot of options were described I assumed it was all there was.
But there are some options like g:matchup_mouse_enabled which are currently NOT described in the README and this was a big no-no when I read in some issue that the plugin has a mouse mapping to override double-clicks 😬.

Please add a few lines about this option at least, to explain that it is enabled by default and can be disabled.