[Docs] Fix relative paths to code (#1662)

* [Docs] Fix relative paths to code

* Update create-a-plugin.md

* Link to existing plugins

* Update README.md

* Update README.md

* Review edits

README.md

@@ -59,11 +59,9 @@ The Backstage platform consists of a number of different components:
 - **app** - Main web application that users interact with. It's built up by a number of different _Plugins_. This repo contains an example implementation of an app (located in `packages/app`) and you can easily get started with your own app by [creating one](docs/create-an-app.md).
 - [**plugins**](https://github.com/spotify/backstage/tree/master/plugins) - Each plugin is treated as a self-contained web app and can include almost any type of content. Plugins all use a common set of platform API's and reusable UI components. Plugins can fetch data either from the _backend_ or through any RESTful API exposed through the _proxy_.
 - [**service catalog**](https://github.com/spotify/backstage/tree/master/packages/backend) - Service that holds the model of your software ecosystem, including organisational information and what team owns what software. The backend also has a Plugin model for extending its graph.
-- **proxy** \* - Terminates HTTPS and exposes any RESTful API to Plugins.
+- [**proxy**](https://github.com/spotify/backstage/tree/master/plugins/proxy-backend) - Terminates HTTPS and exposes any RESTful API to Plugins.
 - **identity** - A backend service that holds your organisation's metadata.
 
-_\* not yet released_
-
 ## Getting started
 
 To run a Backstage app, you will need to have the following installed:
@@ -108,6 +106,7 @@ We would love your help in building Backstage! See [CONTRIBUTING](CONTRIBUTING.m
 - [RFCs](https://github.com/spotify/backstage/labels/rfc) - Help shape the technical direction
 - [FAQ](docs/FAQ.md) - Frequently Asked Questions
 - [Code of Conduct](CODE_OF_CONDUCT.md) - This is how we roll
+- [Adopters](ADOPTERS.md)
 - [Blog](https://backstage.io/blog/) - Announcements and updates
 - [Newsletter](https://mailchi.mp/spotify/backstage-community)
 - Give us a star ⭐️ - If you are using Backstage or think it is an interesting project, we would love a star ❤️