Add notes to `CONTRIBUTING.md`.

notes/CONTRIBUTING.md

@@ -0,0 +1,13 @@
+# Contributing
+
+On Linux, install the following packages:
+
+```bash
+sudo apt install libgdk3.0-cil libatk1.0-dev libcairo2-dev libpango1.0-dev libgdk-pixbuf2.0-dev libsoup-3.0-dev libjavascriptcoregtk-4.1-dev libwebkit2gtk-4.1-dev
+```
+
+Then run:
+
+```bash
+cargo test --workspace --tests
+```