Improve Tempto Documentation

[mattsfuller]

Update documentation for Tempto and give examples of tests, so that others (non-TD) can write tests and contribute to OpenCI.
Tempto README.md does not explain:
How to populate the 'ssh' properties in the configuration file
How to use the new "multiple JDBC table managers" feature.
There are probably other features missing from the readme. I understand example tests have been created for each new feature. That is helpful, but these features need to be documented also.
The readme file is getting rather large. It might be time to split it into multiple files. Maybe one for "quick start", which would explain how to run the sample tests, one for creating Java tests, and another for creating convention based tests.

[mattsfuller]

[ ] Document possible possible header options for query file in convention based tests
[ ] More verbose documentation on defining multiple databases for same table manager type
[ ] Make README more navigable