better dual-audience contributing guidelines

CONTRIBUTING.md

@@ -1 +1,19 @@
-See checklist for [publishing a new release](https://observablehq.com/@observablehq/publishing-new-open-source-releases).
+The Observable runtime is open source and released under the [ISC License](https://github.com/observablehq/runtime/blob/main/LICENSE). We welcome pull requests. If you have a bug report or feature request, please file an [issue](https://github.com/observablehq/runtime/issues). If you want to discuss or ask for help with using the runtime, post in our [forum](https://talk.observablehq.com/) or [community Slack](https://join.slack.com/t/observable-community/shared_invite/zt-1x7gs4fck-UHhEFxUXKHVE8Qt3XmJCig).
+
+## Development
+
+Install dependencies:
+
+```
+yarn
+```
+
+Run tests with Mocha:
+
+```
+yarn test
+```
+
+## For internal use
+
+See our checklist for [publishing a new release](https://observablehq.com/@observablehq/publishing-new-open-source-releases).