[iTowns-dev] Protect master branch

[Vincent Picavet (ml)]

Hello devs,

We changed some rules for contribution to iTowns2. Now the master branch
is protected, and only members of the Reviewers team are allowed to
push. Current members : Alexandr, Gérald, Pierre-Éric, myself.

You therefore have to create branches and pull requests if you want your
code to land in master ( and you really want it :-)

As a general advice, the more you do other's PR code reviews, the more
your PR are likely to be reviewed and merged too !

Do not forget to delete unused and/or obsolete branches in the repository.
You can also fork the repo and create branches on your fork, but this is
not mandatory.  If you do not have commit access to the main repo, you
can fork, or ask an admin to add you in the Core-devs team.

The contributing guide has been updated to reflect these changes.
https://github.com/iTowns/itowns2/pull/135

Have fun,
Vincent