Suggest stressing the importance of enabling *_enabled in docs

[peterkingsbury]

Hello,

First, thanks for an awesome add-on. I'm used to FSM in other languages, and found the godot-finite-state-machine interface rather intuitive.

It however took me a while to track down why the code in my State's _process() wasn't firing. Turns out I needed to explicitly enable one (or more) of:

process_enabled = true
physics_process_enabled = true
input_enabled = true
enter_state_enabled = true
leave_state_enabled = true

I would recommend updating the docs, stressing the necessity of enabling one (or more) of these (as well as the necessary "push down" functions in the entity which contains the FSM instance) so that it is more apparent to newcomers.

Do you accept Pull Requests? I'd be happy to contribute.

Thats all! Thanks for an amazing add-on!