aragonPM: add steps for creating a nested aragonPM instance to reference documentation

[sohkai]

Assuming the user uses APMRegistryFactory (all of our deployed instances do), let's say we want to deploy open.aragonpm.eth off of an existing aragonpm.eth instance:

1. Take the parent aragonpm.eth (this should resolve to an APMRegistry), find its ENSSubdomainRegistrar (.registrar())
2. If not already done, grant an account the ENSSubdomainRegistrar's CREATE_NAME_ROLE
3. With that account, call .createName() on the registrar to create the new name (sha3(open)) and assign ownership to the APMRegistryFactory contract
4. Call .newAPM() on the APMRegistryFactory with the new name (sha3(open)) and tld (namehash(aragonpm.eth)).