docs: add contribution guidelines note to README

Author: robertodauria

README.md

@@ -1,5 +1,11 @@
 # speed.measurementlab.net source code
 
+> **Contributing to this project**
+>
+> We welcome contributions, but please read our guidelines first. If you are interested in contributing - including for Google Summer of Code - we recommend coordinating with a maintainer before starting substantial work. See [our contribution requirements](https://github.com/m-lab/gsoc#2-engage-with-us) for details.
+>
+> We expect all contributions to reflect a genuine understanding of the code being changed. PRs that appear to be generated by AI tools without meaningful human review will be closed.
+
 `mlab-speedtest` is an Angular.js application providing the website https://speed.measurementlab.net. The app code currently uses `gulp` to integrate language localizations prior to deployment.
 
 ## Setting up Local Development Environment