feat: add README

README.md

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+# hax actions
+This repository provides [GitHub
+actions](https://github.com/features/actions) for installing and
+configuring [the hax toolchain](https://github.com/hacspec/hax) and
+its dependencies.
+
+This GitHub action:
+ - Installs the hax toolchain (plus `cargo` and `rustc`);
+ - Installs [F*](https://github.com/fstarlang/fstar) (the version of F\*
+   is frozen and pinned in hax);
+ - Provides the environment variable `HAX_PROOF_LIBS`, `HAX_LIB` and
+   `HACL_HOME`, which are commonly expected by our makefiles that
+   verify hax-extracted code with F*.
+
+By default, the action uses the latest hax on `main` on the repository
+`github:hacspec/hax`. The inputs `hax_repository` and `hax_reference`
+can be set to change this behavior.
+
+Running this action is fast: it takes about a minute and a half (we
+use [cachix](https://app.cachix.org/cache/hax): so commits on the main
+branch of `hax` are pre-built).