Use requirements.txt instead of yaml when buiding from source

README.md

@@ -87,7 +87,7 @@ First time:
 ```bash
 git clone https://github.com/kexinhuang12345/DeepPurpose.git ## Download code repository
 cd DeepPurpose ## Change directory to DeepPurpose
-conda env create -f environment.yml  ## Build virtual environment with all packages installed using conda
+pip install . ## Install DeepPurpose
 conda activate DeepPurpose ## Activate conda environment (use "source activate DeepPurpose" for anaconda 4.4 or earlier) 
 jupyter notebook ## open the jupyter notebook with the conda env