update Contents with new client sections

README.md

@@ -18,6 +18,9 @@ Please contribute to this list to link to all the awesome gemini projects out th
 - [Clients](#clients)
   - [Terminal](#terminal)
   - [Graphical](#graphical)
+    - [Cross-platform](#cross-platform)
+    - [Mobile](#mobile)
+    - [Windows](#windows)
 - [Programming](#programming)
 - [Resources](#resources)
 - [Servers](#servers)