Update README to consolidate & include new BundlerExt features

Updated the README to include new information and to shorten / consolidate things. Removed a few explanatory bits for conciceness, can readd or add to wiki if desired.

Also am in process of updating specs though that PR is not ready yet:

https://github.com/movitto/bundler_ext/commit/ad763cfaec80e93ba5ef23c599f6a1ce580c42fb