Updates to our CONTRIBUTING.md

[AstraBert]

Hey LlamaIndex Community!

We just updated our CONTRIBUTING.md to include guidelines on how to use AI when proposing changes in the LlamaIndex repository.

tl;dr:

AI usage is allowed, as long as you oversaw the development process, understand the code and can maintain it long term

AI can be used to assist you in refactors, testing, writing documentation, but should be avoided when working on core architectural changes or to propose excessively large contributions

You should always start small, validate often, make sure tests pass and quality criteria are met, and build incrementally

We hope this helps the contribution process to be smoother and easier for everyone: happy coding!