Add more detailed configuration example to README

As a result of https://github.com/sanemat/tachikoma/commit/6a23606c33af6ee46e6bfcef943ca0294f533205, the way to configure tachikoma's `root_path` changed.
This commit reflects it.
Merge pull request #2 from sanemat/feature/kyanny-detailed_configuration_example

Add more detailed configuration example to README