Update README to include jcenter dependency

[liuche]

The README doesn't state what dependency to include in gradle in order to use these components. I think there was an email, but it would be great to find either the form of the dependencies to include, or a link to the libraries on jcenter:
http://jcenter.bintray.com/org/mozilla/components/

┆Issue is synchronized with this Jira Task

[pocmo]

The README now links to sub-READMEs that contain the gradle configuration for every component. :)