📝 fix pip install section

You are able to try to install deepchem via pip using the following command.  

However, pip installation is under development, so this command may not work well. 

`deepchem>2.3`

`deepchem=>next_version`

```bash

pip install joblib pandas pillow scikit-learn tensorflow deepchem

```

`deepchem<=2.3`

```bash

pip install joblib pandas pillow scikit-learn==0.22 tensorflow(or tensorflow-gpu)==1.15 deepchem

```

### Install from source

You can install deepchem in a new conda environment using the conda commands in scripts/install_deepchem_conda.sh. Installing via this script will ensure that you are **installing from the source**.  

The following script requires `conda>=4.4` because it uses the `conda activate` command. (Please see the detail from [here](https://github.com/conda/conda/blob/a4c4feae404b2b378e106bd25f62cc8be15c768f/CHANGELOG.md#440-2017-12-20))