https://github.com/steemit/steem/blob/develop/doc/plugin.md
Similarly, external_plugins is set aside for third-party plugins. Just drop plugin code into external_plugins directory, make steemd, and the new plugin will be available.
There is a plugin in example_plugins called hello_api which is a working example of adding a custom API call.
음... 헬로 api 는 아래에서 찾아볼수 있다.
https://github.com/steemit/steem/tree/develop/example_plugins/hello_api
ㅎㅎ