Update README.md with a link to filesystem library

Author: dzenanz

README.md

@@ -28,7 +28,7 @@ Reading and writing image, sound and npz file formats to and from xtensor data s
 mamba install xtensor-io -c conda-forge
 ```
 
-- `xtensor-io` depends on `xtensor` `^0.24.0`.
+- `xtensor-io` depends on `xtensor` `^0.24.0` and [filesystem](https://github.com/gulrak/filesystem).
 
 - `OpenImageIO`, `libsndfile`, `zlib`, `HighFive`, and `blosc` are optional dependencies to `xtensor-io`