[Docs] Fixed some more links (#1648)

docs/plugins/plugin-development.md

@@ -0,0 +1,78 @@
+# Plugin Development in Backstage
+
+Backstage plugins provide features to a 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 APIs and reusable UI
+components. Plugins can fetch data from external sources using the regular
+browser APIs or by depending on external modules to do the work.
+
+<!-- MOVED TO create-a-plugin.md ## 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) -->
+
+## Developing guidelines
+
+- Consider writing plugins in `TypeScript`.
+- Plan the directory structure of your plugin so that it becomes easy to manage.
+- Prefer using the Backstage components, otherwise go with
+  [Material-UI](https://material-ui.com/).
+- Check out the shared Backstage APIs before building a new one.
+
+## Plugin concepts / API
+
+### Routing
+
+Each plugin is responsible for registering its components to corresponding
+routes in the app.
+
+The app will call the `createPlugin` method on each plugin, passing in a
+`router` object with a set of methods on it.
+
+```jsx
+import { createPlugin, createRouteRef } from '@backstage/core';
+import ExampleComponent from './components/ExampleComponent';
+
+export const rootRouteRef = createRouteRef({
+  path: '/new-plugin',
+  title: 'New plugin',
+});
+
+export const plugin = createPlugin({
+  id: 'new-plugin',
+  register({ router }) {
+    router.addRoute(rootRouteRef, ExampleComponent);
+  },
+});
+```
+
+#### `router` API
+
+```typescript
+addRoute(
+  target: RouteRef,
+  Component: ComponentType<any>,
+  options?: RouteOptions,
+): void;
+
+/**
+ * @deprecated See the `addRoute` method
+ */
+registerRoute(
+  path: RoutePath,
+  Component: ComponentType<any>,
+  options?: RouteOptions,
+): void;
+```