Add installation instructions (#20)

Author: bouweandela

README.md

@@ -11,6 +11,12 @@ The data are stored using the data reference syntax structure from DKRZ.
 
 The functionality of this repository is kept simple.
 
+After installing [iris](https://scitools-iris.readthedocs.io/en/latest/installing.html), this package can be installed with the command:
+
+```bash
+pip install ESMValTool_sample_data@git+https://github.com/ESMValGroup/ESMValTool_sample_data
+```
+
 Data can be loaded using the following command for daily or monthly timeseries data.
 
 ```python