Clarify role requirements and restrictions in README

[aeneby]

There are a couple of requirements for this role which could be made clearer in the README:

• the groups thinlinc_agents and thinlinc_masters are in practice required to be present in the inventory
• there must be at least one master server defined in the inventory
• having multiple clusters in the same inventory is not supported

If these requirements are not met, the role will still run but it will result in misconfigured systems (such as orphaned agents, i.e. no master_hostname set).

Another approach could be to use assertions to ensure that these requirements are met.