add guidance for disabled buttons

[sanjaypoyzer]

Clarification for the usage issues outlined in #201 (although not the core issue of contrast).

[gemmaleigh]

I think we should open this one up for discussion.

I'd like to say something along these lines:

Don’t disable buttons, unless there’s a good reason to.

** Describe suitable reason to disable buttons here **

If you have to disable buttons, make sure they look like this...

Any thoughts on a bettter example?

cc. @edwardhorsford, @joelanman, @timpaul.

[gemmaleigh]

I'm summarising from #201:

If you have to disable buttons, make sure they look like you can't click them.

An example of a useful disabled option is a calendar with greyed out days where no bookings are available.

Provide another way for the user to continue, add an error message or text to explain why the button is disabled.

[gemmaleigh]

Closing in favour of #304.