Documentation on requirements missing?

[b1quint]

While dealing with the #112 and #113 issues, I tried to find documentation about the python versions that we support in DRAGONS but I could not find anything. I believe that this kind of information should be described in the README.md file and in the DRAGONS documentation webpage. I believe that now would be a good time to updated this kind of documentation considering that @jehturner is going again through the process of reviewing the required package. I can help with that but I would like to know the team's opinion before writing anything.