Add specific note about not contacting developers privately

[wenzeslaus]

trafficstars

...unless you are offering to pay them.

At least to:

http://ncsu-geoforall-lab.github.io/geospatial-modeling-course/lectures/grass.html#/27

Perhaps this needs to be repeated at a different place and/or generalized for any open source software.

It is good to support students in asking relevant questions on mailing lists etc. but we don't want them to write private messages to developers.