modify git branches structure

To avoid overly frequent commits to master from us (and thus, pulls from master by users), future workflow should be:

• work on feature or bug branch (linked to specific Issue, if relevant)
• merge feature/bug branch into develop branch
• from time to time, merge develop into master, tag and announce new features/bug fixes in newsletter so users can pull
• only commit bug hotfixes directly to master, when needed

To do:

• make develop branch
• make Release Notes in doc (to be used for newsletter)