Add retrofit instructions

Signed-off-by: Adam Harvey <adam.harvey@dxc.com>

.changeset/few-suns-cheer.md

@@ -3,3 +3,26 @@
 ---
 
 Adds "yarn dev" command to simplify local development.
+
+To add the command to an existing application, first add it to the `scripts`
+section of your monorepo root `package.json` like so:
+
+```diff
+ "scripts": {
++    "dev": "concurrently \"yarn start\" \"yarn start-backend\"",
+     "start": "yarn workspace app start",
+     "start-backend": "yarn workspace backend start",
+```
+
+And then add the `concurrently` package to your monorepo, like so:
+
+```sh
+yarn add concurrently@6.0.0 --dev -W
+```
+
+Notes:
+
+- This needs to be done to the monorepo root, not your frontend or backend package.
+- The `--dev -W` will add it only to `devDependencies`, and force it to the monorepo main root.
+
+You can then run `yarn dev` which will start both the Backstage frontend and backend in a single window.