Add documentation for how to get grammars to work with more apps

[dylan-chong]

It is not clear that you must use a regex to match the server-side app window title in order for the grammar/commands to work on a server-side app. I had incorrectly assumed that grammars would work with all apps. Documentation needs to be added to make people aware of this requirement when they make/customise their own grammars.

Just as an example, I am referring to the regex here:

 vim_context = aenea.wrappers.AeneaContext(
     ProxyAppContext(match='regex', title='(?i).*VIM.*'),
     AppContext(title='VIM')
     )