IMOS toolbox use acknowledgement

[ggalibert]

IMOS Admin suggested the following conditions of use could be added to the main Readme.md so that they can measure the impact of the IMOS toolbox in the scientific community. It is based on https://github.com/COSIMA/access-om2#conditions-of-use

Conditions of use

We request that users of this or other IMOS Toolbox code:

1. consider citing ...

2. include an acknowledgement such as the following:

   The authors thank the Integrated Marine Observing System (IMOS; http://www.imos.org.au) for making the IMOS toolbox code available at https://github.com/aodn/imos-toolbox.

TO BE VALIDATED WITH IMOS ADMIN.