Added licence section to README

Signed-off-by: Hugo McNally <[email protected]>

README.md

@@ -2,3 +2,11 @@
 
 This repository contains [Nix](https://nixos.org/) packages and environments used by [lowRISC](https://lowrisc.org).
 These packages and environments are *not supported* by lowRISC or recommended to be used outside of lowRISC.
+
+## License
+
+This package repository is licensed under the [MIT License](LICENSE).
+
+The important clarification made in the [`nixpkgs`](https://github.com/NixOS/nixpkgs/blob/master/README.md#license) repository applies to this repository as well:
+> MIT license does not apply to the packages built by Nixpkgs, merely to the files in this repository (the Nix expressions, build scripts, NixOS modules, etc.).
+> It also might not apply to patches included in Nixpkgs, which may be derivative works of the packages to which they apply.The aforementioned artifacts are all covered by the licenses of the respective packages.