First commit of plugin documentation

plugins/README.md

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+# Plugin Development in Backstage
+Backstage plugins provides features to an Backstage App. 
+
+Each plugin is treated as a self-contained web app and can include almost any type of content. 
+Plugins all use a common set of platform API's and reusable UI components. 
+Plugins can fetch data either from a backend or through any RESTful API exposed directly or through 
+a proxy.
+
+## Creating a new plugin
+On your command-line, invoke the `backstage-cli` to create a new plugin:
+```bash
+yarn create-plugin
+```
+
+![](create-plugin_output.png)
+
+This will create a new Backstage Plugin based on the ID that was provided. It will be built and 
+added to the Backstage App automatically.
+
+*If `yarn start` is already running you should be able to see the default page for your new 
+plugin directly by navigating to `http://localhost:3000/my-plugin`.*
+
+![](my-plugin_screenshot.png)
+
+## Plugin concepts / API
+
+### Routing
+Each plugin is responsible for registering it's components to corresponding routes in the app.
+
+The app will call the `createPlugin` method on each plugin, passing in an `router` object with a set 
+of methods on it.
+
+```typescript
+import { createPlugin } from '@spotify-backstage/core';
+import ExampleComponent from './components/ExampleComponent';
+
+export default createPlugin({
+  id: 'my-plugin',
+  register({ router }) {
+    router.registerRoute('/my-plugin', ExampleComponent);
+  },
+});
+```
+
+#### `router` API
+```typescript
+type RouterHooks = {
+  registerRoute(
+    path: RoutePath,
+    Component: ComponentType<any>,
+    options?: RouteOptions,
+  ): void;
+
+  registerRedirect(
+    path: RoutePath,
+    target: RoutePath,
+    options?: RouteOptions,
+  ): void;
+};
+```