feat: the app plugin

Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Fredrik Adelöw <freben@users.noreply.github.com>
Co-authored-by: Johan Haals <johan@haals.se>
Co-authored-by: Camila Belo <camilaibs@users.noreply.github.com>
Co-authored-by: Camila Belo <camilaibs@gmail.com>
Signed-off-by: blam <ben@blam.sh>

plugins/app/README.md

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+# app
+
+Welcome to the app plugin!
+
+_This plugin was created through the Backstage CLI_
+
+## Getting started
+
+Your plugin has been added to the example app in this repository, meaning you'll be able to access it by running `yarn start` in the root directory, and then navigating to [/app](http://localhost:3000/app).
+
+You can also serve the plugin in isolation by running `yarn start` in the plugin directory.
+This method of serving the plugin provides quicker iteration speed and a faster startup and hot reloads.
+It is only meant for local development, and the setup for it can be found inside the [/dev](./dev) directory.