message: Add catalog population info too
diff: @@ -24,6 +24,31 @@ dummy values just to try out the backend without using the auth or sentry featur
  You may have to issue a `yarn build` command in the project root before doing
  this for the first time.
  +The backend starts up on port 7000 per default.
  +
  +## Populating The Catalog
  +
  +If you want to use the catalog functionality, you need to add so called locations
  +to the backend. These are places where the backend can find some entity descriptor
  +data to consume and serve.
  +
  +To get started, you can issue the following after starting the backend:
  +
  +```bash
  +curl -i \
  + -H "Content-Type: application/json"
  + -d '{"type":"github","target":"https://github.com/spotify/backstage/blob/master/plugins/catalog-backend/fixtures/two_components.yaml"}' \
  + localhost:7000/catalog/locations
  +```
  +
  +After a short while, you should start seeing data on `localhost:7000/catalog/entities`.
  +
  +If you changed the `type` to `file` in the command above, and set the `target`
  +to the absolute path of a YAML file on disk, you could consume your own experimental data.
  +
  +The catalog currently runs in-memory only, so feel free to try it out, but it will
  +need to be re-populated on next startup.
  +
  ## Documentation
  - [Backstage Readme](https://github.com/spotify/backstage/blob/master/README.md)
  