[Docs]: Refresh README to focus on users

[ConnorKirk]

Issue

Currently the repository readme is an architectural overview of the SDK. This is useful for contributors, but not helpful for users interested in using the SDK.

Page/Location

README.md

Suggested Fix

Instead, I think the README should have:

• A description of what this repository is, and what durable executions are
• Link to the developer guide, marketing page and pypi page
• Quickstart example (can copy from existing stuff)
  The architectural overview could be moved elsewhere

[embano1]

Good timing :) #247