Describe permissioned account strategies for `upgradeable-examples`

[cmichi]

We should document how you would implement a strategy for the upgrade-permissioned account in a production system. This should be done in the upgradeable-contracts/README.md.

The typical way would be to either have some governance mechanism for doing that or a set of privileged users that control a multi-signature account.

Substrate provides the proxy and multisig primitives for this purpose.