Make code contributions easier with Gitpod (#2783)

Ref #2772

.gitpod.yml

@@ -0,0 +1,5 @@
+ports:
+  - port: 8080
+tasks:
+  - init: npm install && npm run build
+    command: node server 8080 0.0.0.0

README.md

@@ -107,6 +107,14 @@ You can read a [tutorial on how to add a badge][tutorial].
 4. Run `npm start` to start the server.
 5. Open `http://localhost:3000/` to view the frontend.
 
+Shields has experimental support for [Gitpod Beta][gitpod], a pre-configured development
+environment that runs in your browser. To use Gitpod, click the button below and
+sign in with GitHub. Gitpod also offers a browser add-on, though it is not required.
+Please report any Gitpod bugs, questions, or suggestions in issue
+[#2772](https://github.com/badges/shields/issues/2772).
+
+[![Edit with Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/badges/shields)
+
 To generate the frontend using production cache settings – that is,
 badge preview URIs with `maxAge` – run `LONG_CACHE=true npm run build`.
 
@@ -123,6 +131,7 @@ The server can be configured to use [Sentry][] ([configuration][sentry configura
 Daily tests, including a full run of the service tests and overall code coverage, are run via [badges/daily-tests][daily-tests].
 
 [package manager]: https://nodejs.org/en/download/package-manager/
+[gitpod]: https://www.gitpod.io/
 [snapshot tests]: https://glebbahmutov.com/blog/snapshot-testing/
 [prometheus]: https://prometheus.io/
 [prometheus configuration]: doc/self-hosting.md#prometheus