Add an example file for using `deliverydb` in an standalone way

It's useful to investigate the performance ane behaviour of this individual component.

Usage:

# first, parse the log deliveries into json formatted lines and save them in a file
pushd examples/mailtracking
go run ./main.go -file /path/to/mail.log -workspace /tmp/ws-temp-1 > /tmp/output.json
popd
pushd examples/deliverydb

# use the generated json file with the parsed delivery info as input for storing them into a database
go run ./main.go -workspace /tmp/ws-temp-2 -file /tmp/output.json