add note about performance

> 3. Worse Brotli performance
> As per their own benchmarks, they show that their Brotli implementation is slower than the brotli package. [[ref](https://github.com/developmentseed/starlette-cramjam/tree/main?tab=readme-ov-file#performance)]

ref: https://github.com/stac-utils/stac-fastapi/issues/830

Answer:
Agreed and assumed, performance is not always the main goal. BUT the difference is relatively small, also the size of the result is different and smaller for the cramjam method... which could be faster transfert 🤷

Start with the README's performance section, linked in the issue, and compare its current wording with the referenced Brotli benchmarks. Add a note explaining the relative performance difference and the smaller cramjam result, then verify that the README clearly presents this trade-off.

Written by the indexing model from the issue text.