Add docstring for defstate

[kenrestivo]

I keep forgetting the syntax for this. Rather than having to go diving for the README all the time, would be good to have this vital information. in the docstring.

[arichiardi]

Very good addition :+1:

[tolitius]

agree, thanks!

[NOBLES5E]

@tolitius Can an example usage of docstring be shown in the project README? Otherwise people won't know they can add docstring.

==== Oh sorry I thought this PR means I can add docstring for a state until I read the code.. Is there anyway to add docstring?...

[tolitius]

dev=> (doc mount/defstate)
-------------------------
mount.core/defstate
([state & body])
Macro
  Defines a state. Restarts on recompilation.
   Pass ^{:on-reload :noop} to prevent auto-restart
   on ns recompilation, or :stop to stop on recompilation.

dev=> (defstate answer
        "answer to the ultimate question of life universe and everything"
        :start (+ 1 41))
#'dev/answer
dev=> (mount/start #'dev/answer)
{:started ["#'dev/answer"]}
dev=> answer
42

dev=> (doc answer)
-------------------------
dev/answer
  answer to the ultimate question of life universe and everything

you mean something like this?

[NOBLES5E]

Exactly. Thanks!!