Port config file description to BenchBase wiki.

[lmwnshn]

https://github.com/oltpbenchmark/oltpbench/wiki used to have a little more information about the structure of the config file.

We could probably port and expand on that in the BenchBase wiki.
Additionally, there are some advanced features not seen in most of the default config files that we should document (if they're still around), e.g., https://github.com/oltpbenchmark/oltpbench/blob/master/config/mix_config_postgres.xml the old mixed config.

[timveil]

think this is a great idea and would love to do this once #134 PR is merged as the number and structure of the config files changes.

[timveil]

i'm happy to take this on

[bpkroth]

Related: #370