Move documentation on examples into own README in the examples directory

To put the information closer to where you would be looking for it.
Advertise support for alternative AgentX transports more explicitly

We did already support alternative transports for the AgentX communication
between subagents and master snmpd instances, that is, not just UNIX domain
sockets but eg. also TCP ports. These are specified through transport
specifications that follow the format described in the "LISTENING ADDRESSES"
section in the snmpd(8) manpage.

This change modifies wording that would suggest we supported alternative
UNIX domain socket paths only. It also adds a new script to run simple_agent.py
against a snmpd instance over a TCP port, surprisingly named
"run_simple_agent_over_tcpsocket.sh".