Advertising small changes in organization

[marina-ricci]

• [x] - Rendering notebook when a PR is accepted (see issue #480 )
• [ ] - Closing branch after PR is accepted
• [ ] - Keep the "project" page up-to date by linking the issues
• [ ] - Keep PR in draft mode while it's not ready to be reviewed
• [x] - Stop using "PR ready/not ready" flags
• [ ] - Define a flag for priority PR ?
• [x] - Advertise coverall usage (to find where unit tests are still needed)
• [ ] - Change the "contributing" section accordingly

@m-aguena @combet Please edit/add whatever is relevant

[m-aguena]

For coverage:

• [x] Using coveralls
• [x] Checking for coverage on command line