GOVERNANCE: Update project list to include go-digest

Catching up with opencontainers/tob@ce087c84 9Merge pull request opencontainers#22
from opencontainers/digest-proposal, 2017-01-05).  I'd still rather
drop the parenthetical entirely and link to a place that listed OCI
Projects, but we don't have a canonical target for that yet
(opencontainers/tob#2) and the current closest instance seems to be
the GitHub section in [1] (which doesn't have the "OCI Project"
words).

[1]: https://www.opencontainers.org/community

Signed-off-by: W. Trevor King <wking@tremily.us>

GOVERNANCE.md

@@ -34,7 +34,7 @@ For projects that are not specifications, a [motion to release](#release-approva
 ## Amendments
 
 The [project governance](#project-governance) rules and procedures MAY be amended or replaced using the procedures themselves.
-The MAINTAINERS of this project governance document is the total set of MAINTAINERS from all Open Containers projects (image-spec, image-tools, runC, runtime-spec, and runtime-tools).
+The MAINTAINERS of this project governance document is the total set of MAINTAINERS from all Open Containers projects (go-digest, image-spec, image-tools, runC, runtime-spec, and runtime-tools).
 
 ## Subject templates