doc: add very basic README

Signed-off-by: Roman Volosatovs <[email protected]>

README.md

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+# Description
+
+`west` is a testing framework, which lets you test your WebAssembly applications natively and using all your existing development tools (e.g. debuggers).
+
+## Go
+
+`west` provides `west-bindgen-go` utility, which walks the complete AST of your application and generates a binding file, which replaces functions with `wasmimport` directives, normally provided by the runtime, by a native implementation, which invokes those imports on a WebAssembly component (harness) running in an embedded Wasmtime instance. This lets you test you applications without changing implementation and achieving full test coverage.
+
+# Design
+
+`west` uses [`cabish`](https://github.com/rvolosatovs/cabish) to read and write values over FFI to pass to embedded Wasmtime instance.