add build instructions to README

README.md

@@ -1,20 +1,22 @@
 # ocamlrep: Interop OCaml and Rust code
 
-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 make OCaml code rely on Rust.
+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 should compile with recent versions of the OCaml compiler and Rust ecosystem.
+This project is stand-alone and requires or works with:
 
-ocamlrep requires or works with:
-
-- OCaml 4.14.0
-- Rust 1.65.0 (see Cargo.lock for third-party information)
+  - OCaml 4.14.0;
+  - A rust nightly toolchain.
 
 ## Building ocamlrep
-TODO
+Assume an opam installation on a `4.14.0` like switch. From the repository root,
+```bash
+    OCAMLLIB=$(ocamlopt.opt -config | grep standard_library: | awk '{ print $2 }'); export OCAMLLIB
+    cargo build
+```
 
 ## Contributing
 See the [CONTRIBUTING](CONTRIBUTING.md) file for how to help out.
 
 ## License
-ocamlrep is using the MIT license, as found in the LICENSE file.
+ocamlrep has an MIT license. See the LICENSE file included in this distribution.