jubaconv config documentation

[shiodat]

Using jubaconv with json data, we have to add slash / at the start of value of key.

• default config

"num_rules": [ "key": "name". "type": "num"} 

• jubaconv config with json data

"num_rules": [ "key": "/name". "type": "num"}

This issue occurs because jubatus converts json data using slash.
https://github.com/jubatus/jubatus_core/blob/master/jubatus/core/fv_converter/json_converter.cpp#L96-L110

We should document it.

[kmaehashi]

Fixed via #296