Summary:
Provide a documentation about marketplace feature of Flipper.
This should cover:
1. Introduction of the feature (plugin discovery, auto update)2.
2. Marketplace server - to list available plugins
3. Flipper settings - describe the new settings and how to enable4.
Closes https://github.com/facebook/flipper/issues/3545.
## Changelog
Pull Request resolved: https://github.com/facebook/flipper/pull/4395
Reviewed By: antonk52
Differential Revision: D42918936
Pulled By: passy
fbshipit-source-id: 50b10178b569ecc6ea65b736ea58db401cf686c6
