Add pointers to the more detailed documentation.

Author: youngman-g

docs/README.md

@@ -1,3 +1,11 @@
 This folder contains documentation about the simulator itself.  For
 documentation about the TX-2, please see the [TX-2 project's web
 page](https://tx-2.github.io/).
+
+You might want to read:
+
+- [Design Overview](OVERVIEW.md) - how it works
+- Build instructions for the [CLI tools](build/cli.md) and the
+  web-based emulator (build/web.md)
+- [Documentation about the cross-assembler](assembler), which we use
+  to assemble TX-2 programs.