improve READMEs for ext directory

Author: Forthoney

ext/README.md

@@ -0,0 +1,11 @@
+# Ruby extensions
+`ruby.wasm` uses two C extensions to turn Ruby in to a guest module.
+The `js` extension enables Ruby to use JavaScript APIs. 
+The `witapi` extension exports Ruby's interpreter interface to allow the host to use the Ruby interpreter.
+In other words, `js` allows Ruby to talk to Javascript and `witapi` allows a host to talk to Ruby.
+
+Under each subdirectory, there is a `bindgen/*.wit` file outlining the interfaces for each form of communication. 
+Specifically, `bindgen/rb-js-abi-host.wit` describes embedder's requirements and `bindgen/rb-js-abi-guest.wit` describes exported API from Ruby interpreter.
+The `.c` and `.h` files are autogenerated from [wit-bindgen](https://github.com/bytecodealliance/wit-bindgen#host-runtimes-for-components). 
+You can read more about it in the [contributing guide](/CONTRIBUTING.md#re-bindgen-from-wit-files). 
+Note that we currently do not use the latest version of wit-bindgen because of how fast it is changing, with features being changed or even [removed](https://github.com/bytecodealliance/wit-bindgen/pull/346) at times.