adding loading doc link closes #1803

Author: Lauren McCarthy

src/core/core.js

@@ -51,7 +51,10 @@ var p5 = function(sketch, node, sync) {
    * asynchronous loading of external files. If a preload function is
    * defined, setup() will wait until any load calls within have finished.
    * Nothing besides load calls should be inside preload (loadImage,
-   * loadJSON, loadFont, loadStrings, etc).
+   * loadJSON, loadFont, loadStrings, etc).<br><br>
+   * By default the text "loading..." will be displayed. To make your own
+   * loading page, include an HTML element with id "p5_loading" in your
+   * page. More information <a href="http://bit.ly/2kQ6Nio">here</a>.
    *
    * @method preload
    * @example