Update README.md (google#274)

Reference Gemini more specifically, and describe what's in each of the main 3 directories.

Should help a little with discoverability.

README.md

@@ -1,7 +1,16 @@
-# Google Generative AI Documentation
+# Google Gemini Documentation
 
 These are the source files for the guide and tutorials on
-the [Generative AI developer site](https://developers.generativeai.google/).
+the [Generative AI developer site](https://ai.google.dev/), home to
+the Gemini API and Gemma.
+
+| Path | Description |
+| ---- | ----------- |
+| [`site/`](site/) | Notebooks and other content used directly on ai.google.dev. |
+| [`demos/`](demos/) | Demos apps. Larger than examples, typically consists of working apps. |
+| [`examples/`](examples/) | Examples. Smaller, single-purpose code for demonstrating specific concepts. |
+
+
 
 To contribute to the site documentation, please read
 [CONTRIBUTING.md](CONTRIBUTING.md).