Adding Wouter Knoben's contribution to the ERA5 downloads

[kasra-keshavarz]

Issue description

The download scripts available at here need to acknowledge Wouter Knoben's contributions including parts of the CWARHM workflow.

[ShervanGharari]

It is cleaner to point at the CWARHM repo instead of having the zip file.
Also mentioned Guoqiang as the data on Graham was downloaded by him to my knowledge.
In general sense, the datatool, should be separate from the download script and credit. Users may want to use it in different place with different download tools, or the version of ERA5 they have. Something to consider on where the best place of credit should be.

[kasra-keshavarz]

Solved with #9