Add a development Procfile, document in INSTALL

Since introducing Webpack (commit 66f796c0), there is an additional process required to watch and rebuild Angular app assets.

A separate development `Procfile` is one solution to make things work better _out-of-the-box_, although not necessarily the solution recommended by the maintainer of foreman (see https://github.com/ddollar/foreman/issues/358#issuecomment-17556894).
@kgalli I agree, but I guess a third opinion might be useful – ping @ulferts. thoughts?