Add contribution guide to Hacklang

**Is your feature request related to a problem? Please describe.**
Currently, there is only one guide on how to contribute to **HHVM**. This guide only covers content related to HHVM and not **Hacklang**.

**Describe the solution you'd like**
My proposal is to add a new contribution guide to **Hacklang**.
This guide should contain topics related to configuration, testing and how to build

> I know very well that part of the code related to it is being migrated to **Rust**, which is incredible from my point of view.