misc: Update workflow requirements in CONTRIBUTING.md

Change-Id: I5394ef58930fccea343414964c1fc3e18829d609
Reviewed-on: https://gem5-review.googlesource.com/c/13755
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 2ceded1..465d604 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -77,6 +77,19 @@
 community. Additionally, the maintainer for that part of the code must sign off
 on it.
 
+Contributing long-lived feature branches
+----------------------------------------
+Oftentimes users or institutions add features that are necessarily complex,
+and require many changes on long-lived feature branches. In this case,
+maintaining a perfect history where all changes work individually is infeasible.
+When contributing long-lived feature branches back to gem5's public repository
+users may merge entire long-lived branches into a single changeset and contribute
+their code back as long as 1) the changes have been reviewed by the maintainer
+2) the maintainer agrees to allow such a change, and 3) the changes are passing
+the public tests. Changes that affect common code (outside of a specific
+maintainer's purview) will still need to follow the standard gem5 protocol.
+
+
 Cloning the gem5 repo to contribute
 ===================================