Adding docs for the class directive

[jacwright]

No description provided.

[jacwright]

Fixes #350

[Conduitry]

Thanks for writing some docs to go along with your new feature! I've added a few comments about things I think should be tweaked.

[Cweili]

@jacwright

#371

The demo of the shorthand of class binding has "Error compiling component".

Link:

https://svelte.technology/repl?version=2.14.2&demo=bz2ba6

Error:

Computed property name 'is-selected' is invalid — must be a valid identifier such as is_selected
12:     // Because shorthand relfects the var name, you must use component.set({ "is-selected": true }) or use a computed
13:     // property like this. It might be better to avoid shorthand for class names which are not valid variable names.
14:     "is-selected": ({ isSelected }) => isSelected
        ^
15:   }
16: }