Update README with screenshot

Signed-off-by: Laura Brehm <laurabrehm@hey.com>

Author: laurazard

README.md

@@ -12,6 +12,8 @@
 
 Allows you to quickly jump from a Github repo, branch or PR to a local development environment with the tools you need to test and run your workload — using [Dev Environments](https://docs.docker.com/desktop/dev-environments/) — by adding a button to Github, Bitbucket and Gitlab, in your browser.
 
+<img src="https://github.com/docker/dev-envs-extension/raw/main/docs/button_github_example.png" />
+
 <!-- todo: add marketplace links here-->
 
 Supports Firefox, Chrome and Edge.