chore: Issue 22 - Add app.json to support Heroku Review Apps

PR's from within the same repo now add a link to a deployment

A line gets added like this:

>  armenzg deployed to fx-codecov-staging-pr-23 9 minutes ago    <View deployment>

People starting PRs from other forks will not get automatic deployments.

Review apps are useful for showing work in progress before it gets to master without having to ask someone to deploy it for you.

app.json

@@ -0,0 +1,27 @@
+{
+  "name": "firefox-code-coverage-frontend",
+  "description": "A frontend to show code coverage data",
+  "repository": "https://github.com/armenzg/firefox-code-coverage-frontend",
+  "website": "https://firefox-code-coverage.herokuapp.com",
+  "scripts": {
+  },
+  "env": {
+  },
+  "formation": {
+    "web": {
+      "quantity": 1,
+      "size": "standard-1x"
+    }
+  },
+  "addons": [
+
+  ],
+  "buildpacks": [
+    {
+      "url": "heroku/nodejs"
+    },
+    {
+      "url": "https://github.com/hone/heroku-buildpack-static"
+    }
+  ]
+}