So, you want to help this project. Good news, you: I need help here!
There are lots of ways you help, and here are some examples and guidelines; take them more as suggestions than required steps, though. (Note: you're also welcome to contribute to this file, proposing changes or other examples. So meta!)
Found a bug in the code you want fixed? Sure, let's get into it!
The first thing you should do is try to come up with a MWE that demonstrates the issue. After all, you could be working on a large codebase as far as I know, and checking thousands of lines of code would be impractical for me.
After that, tell me exactly what went wrong and what should be happening. Please, show me every bit of math or code you think is relevant (and keep in mind: I'm not an expert in anything, you may have to teach me a thing or two). While you're at it, please recheck your math and make sure that my code is the one doing the wrong thing.
Finally, submit an issue here on GitHub, making sure to report all the needed details to reproduce the problem (and, if relevant and possible, come up with a unit test, so this issue won't come again in the future).
After submitting the issue, if you want to solve it yourself, please, feel free to submit a pull request. More on this in the Solving Issues section bellow.
Had an idea to improve usability, performance or documentation? Want to talk about a feature you wish was implemented? Go on, file an issue here.
I may need to ask you a few things before working on it, and we may have to discuss what would that change affect, but worry not: I'm not that bad of a person to talk to, once you get used to my mumblings that is.
And remember: breaking changes are allowed to happen in major releases only (and in the 0.x series, but that's a special case), so your idea may need to be delayed.
Found an issue you want to work on? Great! Let's get to work!