add notes about Middleware being experimental

the Middleware adds a `rasterio.Env()` block and might not be compatible when the API has other `rasterio.Env()` blocks (meaning multiple levels of Env).

This might also be specific to Readers (rio-tiler) using multithreading 🤷‍♂️ 

I encountered this in https://github.com/developmentseed/titiler/blob/master/titiler/endpoints/factory.py#L904