Move/Include directions on how to contribute a simple eval into top-level README.md

[jonathanagustin]

I feel like one primary aim of this project is to receive high quality evals.

Instead of the docs folder, I propose that the top-level evals/README.md should include directions on how to contribute a simple eval.

I don't propose the entirety of evals/docs/build-eval.md should be moved into evals/README.md; however, I propose something minimal and simple be included.

I recognize it isn't a big effort to go to evals/docs/build-eval.md but I feel like if eval contribution is a primary aim, directions should be at the forefront (versus the periphery).