update examples with difficulty stars

examples/core/core_drop_files.c

@@ -2,6 +2,8 @@
 *
 *   raylib [core] example - Windows drop files
 *
+*   Example complexity rating: [★★☆☆] 2/4
+*
 *   NOTE: This example only works on platforms that support drag & drop (Windows, Linux, OSX, Html5?)
 *
 *   Example originally created with raylib 1.3, last time updated with raylib 4.2