chore: add doc for martkeplace (#4395)

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

docs/tutorial/marketplace.mdx

@@ -0,0 +1,38 @@
+---
+id: marketplace
+title: Marketplace
+---
+import useBaseUrl from '@docusaurus/useBaseUrl';
+
+Flipper provides this feature to allow distribution of `private` plugins, which are not published to [npmjs.com](https://npmjs.com).
+
+### Plugin discovery
+
+Plugins which are supported by currently connected app/device, will be listed under "Detected in app" section in sidebar.
+This significantly help users to discover useful plugins for their app.
+
+<img alt="Install plugins" src={useBaseUrl("img/marketplace-discovery.png")} />
+
+### Plugin auto-update
+Once plugin is installed, it will also keep updating to latest version. 
+
+
+## Enable markektplace
+
+### Setup server
+
+Instead of hosting plugins on npm server, we need to provide custom server, which will provide a list of internal plugins.
+
+> It is recommended to connect this server with internal package registry (to allow providing latest version).
+
+Here is a example implementation of server using Verdaccio as package registry: [marketplace-server-example](https://github.com/usrbowe/flipper-marketplace-server).
+
+
+### Enable marketplace
+
+Open Flipper settings and enter the URL to your Flipper marketplace server. It is also adviced to toggle ON the `Enable auto update`.
+
+<img alt="Install plugins" src={useBaseUrl("img/marketplace-settings.png")} />
+
+
+