readme: update build instructions

README.md

@@ -8,7 +8,23 @@ It checks [HackerNews](http://news.ycombinator.com), [Reddit](http://www.reddit.
 
 `$ go build`
 
+### Plugins
+
+```shell
+$ cd plugins/golang_news
+$ go build -buildmode=plugin
+```
+
 ## Run
 
 Add your credential of your personal twitter bot to `settings.json`, look at
 `settings.json.example` for, well, an example.
+
+The settings file is expect to be in the same directory as the executable.
+
+Finally run the binary with the first argument pointing to the plugin (`*.so`
+file):
+
+```shell
+$ ./golang_news plugins/golang_news/golang_news.so
+```