The contributor guide should be updated to send features to 2.7

[stof]

Any new backward-compatible feature should be submitted to the 2.7 branch (which is the default branch now), not to the master branch (which is for 3.0).
But because of the documentation saying to send features to master (or at least I suppose it is because they read the doc), many recent PRs have been submitted to master rather than the default branch 2.7.

IMO, we should update the contributor guide to take into account the fact that 2.7 and 3.0 are being developed in parallel

[wouterj]

I agree