update stream naming policy
    
    based on the following decision at the Modularity Team meeting:
    
    There will be two streams named "rolling" for the user-focused builds and "unstable" for the preview/master/pre-release things, and this will be just a suggestion to the packagers, but using whatever is established upstream is also a good option. We'll not rename nor enforce renaming of anything, we'll leave it to packagers and mandate a good use of the description summary field for this purpose. +4 0 -0