Document Git access and fork-and-PR contribution model

getting-started.md

@@ -58,6 +58,13 @@ During the pilot phase, Nathan Schneider serves as General Manager and holds
 final discretion — but members deliberate now, and full member governance is the
 goal as the co-op grows.
 
+### Help build it
+
+Everything we run is open source, and members are welcome to contribute code,
+documentation, and design. See [Infrastructure](infrastructure.md) for the map
+and the "Contributing" section for how to propose changes through
+[git.inference.coop](https://git.inference.coop).
+
 ## Your allowance
 
 Membership includes **$15 of usage credit each month**, shared between chat and