misc: Updated CONTRIBUTING.md to discuss releases and hotfixes

A new section in CONTRIBUTING.md has been added to discuss the
proceedure for how releases are carried out, as well as hotfixes.

Jira: https://gem5.atlassian.net/browse/GEM5-297
Change-Id: I49e7d6e41e8a6d5387c839eb26263e86dd52c294
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24843
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed