Mention preference for overloads in contrib doc

Author: armanbilge

CONTRIBUTING.md

@@ -54,6 +54,7 @@ Please:
 * Use `def` for read-only properties unless there is a compelling reason it should be a `val`
   (i.e., the spec definitively states it is constant)
 * Use `Double` for integer-values that can fall outside the range of `Int`
+* Prefer adding overloads instead of using union `|` types for method and constructor parameters
 * Add scaladocs via copy-paste from MDN