misc: Updated CONTRIBUTING.md to reflect altered release policy

misc: Updated CONTRIBUTING.md to reflect altered release policy

It has been decided that contributions can be made to the staging branch
(assuming they are of a high enough importance). The staging branch will
then be merged into both the master and develop branches.

The time in which the staging branch exists has been extended to two
weeks.

Jira: https://gem5.atlassian.net/browse/GEM5-334
Change-Id: I3cd0b344be9768871b7fd79261c603d17d8ac1b8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25523
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>