Document how to contribute to Layerform

[lucasfcosta]

With new contributors coming in, we should write a proper CONTRIBUTING.md file to guide new developers and offer then a good experience.

Here are a few things I think we should include:

• [ ] How to install the Layerform CLI locally
• [ ] How to run the Layerform CLI straight from source go run main.go
• [ ] What the examples do and how to run them
• [ ] Brief guidelines on how the internals of the project work and where important files/functions are
• [ ] Code standards (go fmt etc)