added mamba instructions, which installs quickly

Author: sarthakpati

docs/setup.md

@@ -10,8 +10,9 @@
 ```powershell
 conda create -n venv_gandlf python=3.6 -y
 conda activate venv_gandlf
+conda install -c conda-forge mamba # allows for faster dependency solving
 conda install -c pytorch pytorch torchvision -y # 1.8.0 installs cuda 10.2 by default, personalize based on your cuda/driver availability via https://pytorch.org/get-started/locally/
-conda install -c conda-forge gandlf
+mamba install -c conda-forge gandlf -y
 ```
 
 **Note for Windows users**: Please follow instructions for [developers](./extending).