add tip for the JSON output

[madneal]

This PR provides a tip when output as JSON due to the current default pretty-print flag

Current Issue

For the orginal json flag provided by checker.go, it's impossible to get analysisflags.JSON from nilaway. There is a conflict bettwen the pretty-print and json output currently due to the code https://github.com/uber-go/nilaway/blob/e90288479601315af13b7fdd3ccd6b50c53a8e7c/nilaway.go#L43. As the default conf.PrettyPrint is true, so the output will be messy when try to output as json.