Add LlamaIndex to awesome-transformers.md
# What does this PR do?

This PR adds `LlamaIndex` to `awesome-transformers.md`. `LlamaIndex` is a project that provides a central interface to connect your LLM's with external data.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

_The documentation is not available anymore as the PR was closed or merged._<|||||>Interestingly, you have `transformers` as an optional dependency when the Python version is above 3.9. It seems to be used quite a bit for tokenizing/streaming/image captioning etc. 

If you'd rather not have it as a main dependency, I'd love to hear why so that we may improve! We've been trying to keep the package on the lighter side of dependencies so that it wasn't too heavy to add.

Thank you!