Update template to add references

Added a new section for reference, such as ML threads, or links
to design summit notes.

At the same time, ensuring authors take care that the document
is readable without access to any of the references.

Based on an idea suggested here:
https://review.openstack.org/#/c/81932

Change-Id: Ifcc69ffb4e9859ad3138b7e2acc716b97d232f39
Merge "Update template to add references"