readme updates (#58)

Summary:
remove circleci badge, add github actions badges, update toolchain versions.

Pull Request resolved: #58

Reviewed By: capickett

Differential Revision: D53045996

Pulled By: shayne-fletcher

fbshipit-source-id: 238fcfdf78c1f5a845a7642766199491d9d85bad

README.md

@@ -1,18 +1,18 @@
-# ocamlrep: Interop OCaml and Rust code [![facebook](https://circleci.com/gh/facebook/ocamlrep.svg?style=svg)](https://app.circleci.com/pipelines/github/facebook/ocamlrep)
+# ocamlrep: Interop OCaml and Rust code [![Buck build and test](https://github.com/facebook/ocamlrep/actions/workflows/buck-build-and-test.yml/badge.svg)](https://github.com/facebook/ocamlrep/actions/workflows/buck-build-and-test.yml) [![Cargo build and test](https://github.com/facebook/ocamlrep/actions/workflows/cargo-build-and-test.yml/badge.svg)](https://github.com/facebook/ocamlrep/actions/workflows/cargo-build-and-test.yml)
 
 The goal of this project is to make OCaml and Rust code interoperable. While in its early stage of development, this library is actively used by the [HHVM](https://github.com/facebook/hhvm) project, mostly in the Hack type checker, to enable OCaml code to rely on Rust.
 
 ## Requirements
-This project is stand-alone and requires or works with:
-  - OCaml 4.14.0;
-  - A rust nightly toolchain.
+This project is stand-alone and is currently tested with:
+  - OCaml 5.1.1;
+  - The Rust toolchain in [./rust-toolchain](rust-toolchain)
 
 ## Building ocamlrep
 
 There are two build methods. One way is to use [Cargo](https://doc.rust-lang.org/cargo/guide/cargo-home.html) and the other is to use [Buck2](https://buck2.build/).
 
 - Cargo: Install OPAM then `cargo build`.
-- Buck2: See this guide for [Building ocamlrep with Buck2](README-BUCK.md).
+- Buck2: See [Building ocamlrep with Buck2 and Reindeer](README-BUCK.md).
 
 ## Contributing
 See the [CONTRIBUTING](CONTRIBUTING.md) file for how to help out.