hacking: described new branching policy for 1.12 an later

hacking: described new branching policy for 1.12 an later

This change closes automake bug#11153.

* HACKING (Working with git): Document the new policy for automake
branching and merging, which will start being applied after 1.12 is
released:

  - the maintenance branch will be 'maint', and we will cut the
    maintenance releases directly from there;

  - the development branch will be 'master', and we will cut the
    new releases directly from there;

  - 'maint' will be kept regularly merged into 'master'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>