Provide a CONTRIBUTING.md file

[miguelcrespo]

Hi, I was checking out the code of this plugin since I use it a lot and I wanted to understand the code and I notice there's no CONTRIBUTING.md, it might be beneficial to include one to give a brief introduction of how to contribute to this project, some sections I have in mind:

• Introduction to the project and architecture
• Code standards
• How to run the project locally and start developing

[ten3roberts]

That is a good idea and would further help Neogit becoming an active project.

I'll also add for myself to remember that the todo.txt could be removed as it is mow far from relevant