Units policy missing

Migrate https://wiki.ubuntu.com/UnitsPolicy

Notes:
- This should also be used for consistency checking across the docs set (mention in contrib. guide).
- The rule is (which the wiki page doesn't mention): _"when packaging a piece of sw, the maintainer needs to ensure the sw reports the units this way" (It's why we patch a bunch of software to use the correct units required by the policy)_