[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

CONTRIBUTING.md

@@ -54,6 +54,10 @@ If you are proposing a feature:
 - Remember that this is a volunteer-driven project, and that contributions
   are welcome :)
 
+## Add your company to ADOPTERS
+
+Have you started using Backstage? Adding your company to [ADOPTERS](ADOPTERS.md) really helps the project.
+
 # Get Started!
 
 So...feel ready to jump in? Let's do this. Head over to the [Getting Started guide](https://github.com/spotify/backstage#getting-started) 👏🏻💯