docs: added RunIt README.md

packages/run-it/README.md

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+# RunIt
+
+**RunIt** is a React-based specification-driven REST API request tester.
+
+Looker's [API Explorer](https://github.com/looker-open-source/sdk-codegen/tree/main/packages/api-explorer) uses the **RunIt** package for testing API endpoints.
+
+A configuration provider interface enables **RunIt** to work both in the [Looker Extension Framework](https://docs.looker.com/data-modeling/extension-framework/extension-framework-intro) runtime environment and as a stand-alone browser application.
+
+OAuth and CORS [are used](https://github.com/looker-open-source/sdk-codegen/blob/main/docs/cors.md) to authenticate and submit REST requests for the stand-alone version.
+
+## RunIt tabs
+
+The [RunIt](src/RunIt.tsx) component has multiple tabs:
+
+### Request
+
+The Request tab displays a form for entering inputs to the chosen REST API endpoint
+
+### Response
+
+API responses are displayed on this tab. Support for rendering a variety of API response types like as JSON, CSV, Markdown, JPEG, HTML, PNG, and SQL are supported.
+
+### SDK Call
+
+The language SDK calling syntax provided by the Looker codegen project (also in this repository) is shown on this tab.
+
+### Performance
+
+Only for the stand-alone version, some performance information for HTTP request and response processing is shown here.
+
+### Configuration
+
+Only for the stand-alone version, this tab is used to configure the OAuth server for API request tokens.
+
+## Using RunIt
+
+See the [stand-alone API Explorer](https://github.com/looker-open-source/sdk-codegen/tree/main/packages/api-explorer/src/StandaloneApiExplorer.tsx) and the [extension version of API Explorer](https://github.com/looker-open-source/sdk-codegen/tree/main/packages/extension-api-explorer/src/ExtensionApiExplorer.tsx) for reference implementations.