I assume you're reading this doc because you want to help improve NGraphics. That's great! Thank you!
But before you get started, please note the following:
The official code formatting follows mono.
Basically, it goes against the default Visual Studio style and is darned close to MonoDevelop's defaults.
If you submit a pull request with formatting changes in parts of code not otherwise edited, the PR will be rejected.
For tabs, I prefer actual tab characters. You will notice the codebase bounces back and forth between the two since I keep getting caught up in other people's holy wars.
Same goes for line endings - some are Windows style while others are correct.
If either of these bothers you, upgrade your text editor.
Don't.
If you add a significant feature, there needs to be some test coverage of that code. I don't care about 100% coverage, but I need proof your code works.