update README with link to spec

[samlaf]

Added some spec-like diagrams and descriptions into the README in #233

Did not know that we had a "spec-like" integration guide. Added a link to it at the top of the README.

Currently also working on an actual spec for integrations. We should probably remove every "spec-like" thing in the README, and make sure the integration guide and spec don't overlap too much.

Action Item:

1. add spec link to README once it's ready (and added to docs.eigenda website)