Create CONTRIBUTING.md

[unrealapex]

I noticed that there is no documentation regarding a few things:

• how to contribute to this project
• expectations
• testing code using the minimal config files in debug/
  • Test snippets with the minimal debug config using nvim -u <debug-file>
• style guidelines
  • what JSON formatter to use
  • ensuring commits meet the Conventional Commits specification
• links resources on how to create snippets
  • VS Code documentation
  • Lua snippets
  • etc...

Adding these to a CONTRIBUTING.md would making contributing to this project more streamlined.

[OkelleyDevelopment]

Fair point, perhaps this is something @rafamadriz can weigh in on?