Explain Bower usage for FxA in README.md

[johngruen]

From a contributor email:

Hi everyone,

don't know if this is the right place to ask - but if it is, would somebody kindly include a hint on bower and how to handle it's implications in the README?

After getting and installing a fresh clone from git, there are no bower_components in the apps directory. Took me quite a while to figure out how to handle this. Maybe I'm even wrong by now...

Anyway, I just can't tell how glad I am to see that it's still possible to sync FF locally. Thanks a lot to all of you!

[vladikoff]

fixed via https://github.com/mozilla/fxa-content-server/blob/master/CONTRIBUTING.md#bower Thanks!