Documented mg_send_file()

docs/API.md

@@ -169,6 +169,15 @@ a response body.  Mongoose provides functions for all three parts:
      send data in pieces by saving state in
      `struct mg_connection::connection_param` variable and returning `0`. Then
      Mongoose will call a handler repeatedly after each socket write.
+     
+     void mg_send_file(struct mg_connection *, const char *path);
+     
+Signals Mongoose to serve given file. Mongoose handles file according to
+it's extensions, i.e. Mongoose will invoke CGI script if `path` has CGI
+extension, it'll render SSI file if `path` has SSI extension, etc. If `path`
+points to a directory, Mongoose will show directory listing. If this function
+is used, no calls to `mg_send*` or `mg_printf*` functions must be made, and
+event handler must return `MG_MORE`.
 
 <!-- -->