CONTRIBUTING: Allow collaborative pull requests
CONTRIBUTING: Allow collaborative pull requests

For runtime-spec, there are often PRs that pickup and reroll another
user's commits (e.g. opencontainers/runtime-spec#337). As long as the
Signed-off-by entries are there (for the DCO) and the new PR
references the earlier work (to avoid maintainer confusion), I see no
problem with this sort of collaboration.

I thought about replacing the old wording with words like the above
paragraph, but it seemed overly prescriptive.

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

xiekeyang/image-tools,opencontainers/image-tools,xiekeyang/image-tools,opencontainers/image-tools