adding rollbar plugin & backend

This is the first commit of the rollbar plugin. It is fully functional
but will require some additional features to make it more usable and
feature complete. It currently includes a backend wraps the rollbar REST
API and provides data for the frontend plugin.

plugins/rollbar-backend/README.md

@@ -0,0 +1,12 @@
+# Rollbar Backend
+
+Simple plugin that proxies requests to the [Rollbar](https://rollbar.com) API.
+
+## Setup
+
+A `ROLLBAR_TOKEN` environment variable must be set to a read access account token.
+
+## Links
+
+- (Frontend part of the plugin)[https://github.com/spotify/backstage/tree/master/plugins/rollbar]
+- (The Backstage homepage)[https://backstage.io]