readme: add initial pkgcraft-related info

Author: radhermit

README.md

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+[![ci](https://github.com/pkgcraft/bash/actions/workflows/test.yml/badge.svg?branch=main)](https://github.com/pkgcraft/bash/actions/workflows/test.yml)
+
+# bash
+
+Forked version of bash supporting shell interactions (e.g. writing builtins or
+modifying variables, arrays, and functions) natively in rust via the scallop crate.
+
+## Development
+
+Note that the development workflow involves rebasing against upstream and force
+pushing to keep the patch stack in order.