packages: add READMEs

packages/app/README.md

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+# @spotify-backstage/app
+
+This package is an example of a Backstage application.

packages/cli/README.md

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+# @backstage/cli
+
+This package provides a CLI for developing Backstage plugins and apps.
+
+## Installation
+
+Install the package via npm or yarn:
+
+```sh
+$ npm install --save @backstage/cli
+```
+
+or
+
+```sh
+$ yarn add @backstage/cli
+```
+
+## Documentation
+
+- [Backstage Readme](https://github.com/spotify/backstage/blob/master/README.md)
+- [Backstage Documentation](https://github.com/spotify/backstage/blob/master/docs/README.md)

packages/core/README.md

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+# @backstage/core
+
+This package provides the core API used by Backstage plugins and apps.
+
+## Installation
+
+Install the package via npm or yarn:
+
+```sh
+$ npm install --save @backstage/core
+```
+
+or
+
+```sh
+$ yarn add @backstage/core
+```
+
+## Documentation
+
+- [Backstage Readme](https://github.com/spotify/backstage/blob/master/README.md)
+- [Backstage Documentation](https://github.com/spotify/backstage/blob/master/docs/README.md)

packages/storybook/README.md

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+# storybook
+
+This package provides a storybook build for Backstage. See [storybook.backstage.io](http://storybook.backstage.io)