README: Add note about npm install/yarn root

[addyosmani]

Minor one. When you clone sw-helpers, it's not clear right now what the expectation would be when you npm install or yarn root.

One might guess this will recursively try installing all the dependencies for the sub-packages in the mono-repo. Or just the deps needed to hack on sw-helpers, but since we don't appear to note this in the readme it's unclear.

[addyosmani]

Further feedback:

• We should tell folks that they'll need to run gulp in order to build the packages
• Might be useful to note whether you can individually build a subpackage and how (e.g do you need to build everything as they rely on each other as dependencies)

[gauntface]

@addyosmani I added this to the docs: https://googlechrome.github.io/sw-helpers/contributing.html#main

I think this addresses the above issues, let me know if not.