Addition of hello_notify

This worker is an example of how to transmit NOTIFY messages using a
background worker, that can be LISTEN'ed by other backends at will.
This example uses a simple method based on pg_notify to send requests
with a payload based on the data of pg_stat_activity.

NOTIFY messages are sent to backends who latest running query takes
more than the naptime worker sleeps at each execution loop. NOTIFY
channel, as well as the database where messages are sent can be
customized. Only the database name cannot be changed after startup,
all the other parameters can be reloaded at will.

Basic idea and implementation by legendetm (Raido), with a fair
amount of additional design, bug fixes and corrections by me.
Update README with more general information about contributors