Document npm local vs global installation of punch and/or plugins

[adrinux]

I ran into some issues installing a punch plugin laktek/punch-sass-compiler#1 which I've been unable to replicate. None the less I think some mention of npm's ability to do local as well as global package installs would be useful.

It might also be useful to discuss various approaches to installing punch and plugins and the pros and cons of local vs global install location.

This helped me understand npm Local vs Global:
'Where does npm put stuff?' and following answers at https://npmjs.org/doc/faq.html