Document the process to pin the constraint in tox.ini

As discussed in Victoria PTG[1], we need to start pining
the constraint in tox.ini on Tempest new release so that our
tox.ini can match with compatible constraints with Tempest version.

[1] https://etherpad.opendev.org/p/qa-victoria-ptg

Change-Id: Ib705f59ef3aea93b2778cc87fc44d390b5a62c94
2 files changed