Adds more information to README.md
Adds more information to README.md

- Expands the introduction, best practices and why it's faster.
- Adds the Caveat section.

Change-Id: I0fef7e7a0bc69289046f9b0d1d128b872f9ecd08

ampproject/amp-toolbox,google/amp-toolbox,google/amp-toolbox,google/amp-toolbox,ampproject/amp-toolbox,ampproject/amp-toolbox,ampproject/amp-toolbox