message: docs/backend-system: add plugin creation section
diff: @@ -16,6 +16,12 @@ Backend [plugins](../architecture/04-plugins.md) and
  backend _features_, are the building blocks that adopters add to their
  [backends](../architecture/02-backends.md).
  +## Creating a new Plugin
  +
  +This guide assumes that you already have a Backend project set up. Even if you only want to develop a single plugin for publishing, we still recommend that you do so in a standard Backstage monorepo project, as you often end up needing multiple package. For instructions on how to set up a new project, see our [getting started](../../getting-started/index.md#prerequisites) documentation.
  +
  +To create a Backend plugin, run `yarn new`, select `backend-plugin`, and fill out the rest of the prompts. This will create a new package at `plugins/<pluginId>-backend`, which will be the main entrypoint for your plugin.
  +
  ## Plugins
  A basic backend plugin might look as follows:
  