Update README.md

    - xarray

```

On Unix/Linux/Mac, create the environment using

With conda installed, create the environment using

```bash

conda env create --file dm-tools_py3.yml

### Activate the environment

On Unix/Linux/Mac:

After creating the environment, activate it using:

```bash

source activate dm-tools_py3

### De-activate the environment

On Unix/Linux/Mac:

To de-activate the environment, use:

```bash

source deactivate