update conda install info (#395)

thanks. Also addresses #392

INSTALL.md

@@ -9,14 +9,20 @@ INSTALL file for Faiss (Fair AI Similarity Search)
 Install via Conda
 -----------------
 
-The easiest way to install FAISS is from anaconda. We regularly push stable releases to conda channel. FAISS conda package depends on mkl package shipped in conda.
+The easiest way to install FAISS is from anaconda. We regularly push stable releases to conda channel. FAISS conda package depends on mkl and numpy package shipped in conda.
+
+Currently we support faiss-cpu on both Linux and OSX platforms. We also provide faiss-gpu compiled with CUDA8.0/CUDA9.0/CUDA9.1 on Linux systems.
+
 You can easily install it by
 
 ```
 # CPU version only
 conda install faiss-cpu -c pytorch
-# GPU version requires CUDA to be installed, otherwise it falls back to CPU version
-conda install faiss-gpu -c pytorch
+# Make sure you have CUDA installed before installing faiss-gpu, otherwise it falls back to CPU version
+conda install faiss-gpu -c pytorch # [DEFAULT]For CUDA8.0, comes with cudatoolkit8.0
+conda install faiss-gpu cuda90 -c pytorch # For CUDA9.0
+conda install faiss-gpu cuda91 -c pytorch # For CUDA9.1
+# cuda90/cuda91 shown above is a feature, it doesn't install CUDA for you.
 ```
 
 Compile from source