Minor release policy update

[Richard Frith-Macdonald]

I have updated the release policy document on the wiki slightly ...  
http://wiki.gnustep.org/index.php/GNUstep_release_policy

The idea is just to make the stable branches/releases more obvious by  
using the word 'stable' rather than the version numbers as the name  
for the branch.
This also means we can more easily automate processes for the stable  
branch ... I've made a tiny start by adding a gnustep-make 'svn- 
bugfix' target to export a .tar.gz archive from the stable branch.