Watch command: improvements

* I just realized I missed an error, this is fixed in https://github.com/Chive/hyhyhy/commit/4aba7ece75f430b5816677bfb2750956529f7ad8
* Also, the watch command now runs build after running for a first time automatically - seemed annoying when you did changes and run ``hyhyhy watch`` afterwards and it didn't update.
* I lastly added some documentation.