Adding an /llms.txt file for OPTIMADE

[ml-evs]

/llms.txt (https://github.com/AnswerDotAI/llms-txt) is an attempt to standardize the way in which APIs and libraries can describe themselves so that LLM agents can make the best use of them, especially when such libraries are changing more frequently than the training sets of the LLMs themselves. It can be used in the same manner as robots.txt on a website, like a documentation site or specification.

I think this would be worthwhile for us to add for OPTIMADE, and it could be encouraged that individual providers either link to it, or host their own versions too. One some other projects, I have also found that this kind of description also meshes quite nicely with what a beginner would want to see, so we can maybe kill two birds with one stone by writing a brief user guide for OPTIMADE that can double up as an llms.txt.

Relatedly, one thing that I am often asked for is examples for OPTIMADE queries. Given that this /llms.txt format should probably also include such a set of examples, we could consider adding these as a separate document or as part of the specification, which can then be directly copied into the /llms.txt.