Document how/where mixins are installed

[carolynvs]

I need to document how plugins are installed in PORTER_HOME (the summary is ~/.porter/mixins/NAME/NAME, e.g. ~/.porter/mixins/exec/exec where exec is a binary, and exec-runtime is next to it and is a linux binary).

[carolynvs]

This is going to change eventually in PEP005 - Mixins are Bundles. When we have that we won't "install" mixins anymore, just pull and run like a docker image.

But we still need some documentation about the mixins directory until that happens.