Add contributing note to dev server packages (#39127)

Summary:
Pull Request resolved: #39127

Will be referenced in react-native-community/cli#2055.

Changelog: [Internal]

bypass-github-export-checks

Reviewed By: motiz88

Differential Revision: D48603224

fbshipit-source-id: ac98565cf2f739e02f3d552522fa7ab39ebb9771

Author: huntie

packages/community-cli-plugin/README.md

@@ -85,3 +85,7 @@ Accepts all options supported by [`bundle`](#bundle) and the following:
 | Option | Description |
 | - | - |
 | `--indexed-ram-bundle` | Force the "Indexed RAM" bundle file format, even when building for Android. |
+
+## Contributing
+
+Changes to this package can be made locally and tested against the `rn-tester` app, per the [Contributing guide](https://reactnative.dev/contributing/overview#contributing-code). During development, this package is automatically run from source with no build step.

packages/dev-middleware/README.md

@@ -80,3 +80,7 @@ WebSocket handler for registering device connections.
 #### `/inspector/debug`
 
 WebSocket handler that proxies CDP messages to/from the corresponding device.
+
+## Contributing
+
+Changes to this package can be made locally and tested against the `rn-tester` app, per the [Contributing guide](https://reactnative.dev/contributing/overview#contributing-code). During development, this package is automatically run from source with no build step.