doc(poster.go): added comment for post details

  Added comment of a link to were the Post() function came from.

poster.go

@@ -6,6 +6,7 @@ import (
 	"net/url"
 )
 
+// https://zetcode.com/golang/getpostrequest/ post_req_form.go
 func Post(where string, data *url.Values) *http.Response {
 	resp, err := http.PostForm(where, *data)
 	if err != nil {