Improve installation doc with conda

Closes #390.

README.md

@@ -28,7 +28,7 @@ xFormers is:
 
 ## Installing xFormers
 
-* **(RECOMMENDED) Using binaries**: We provide binaries for Linux and recent PyTorch versions. Install xFormers with conda:
+* **(RECOMMENDED) Using binaries**: We provide binaries for Linux and recent PyTorch versions. After you have [installed pytorch in conda](https://pytorch.org/get-started/locally/), install xFormers with conda:
 
 ```bash
 conda install xformers -c xformers/label/dev
@@ -219,6 +219,7 @@ Patrick et al., 2021](https://arxiv.org/abs/2106.05392)*
 * the version of GCC that you're using matches the current NVCC capabilities
 * the `TORCH_CUDA_ARCH_LIST` env variable is set to the architures that you want to support. A suggested setup (slow to build but comprehensive) is `export TORCH_CUDA_ARCH_LIST="6.0;6.1;6.2;7.0;7.2;7.5;8.0;8.6"`
 * If the build from source OOMs, it's possible to reduce the parallelism of ninja with `MAX_JOBS` (eg `MAX_JOBS=2`)
+* If you encounter [`UnsatisfiableError`](https://github.com/facebookresearch/xformers/issues/390#issuecomment-1315020700) when installing with conda, make sure you have pytorch installed in your conda environment, and that your setup (pytorch version, cuda version, python version, OS) match [an existing binary for xFormers](https://anaconda.org/xformers/xformers/files)
 
 
 ### License