BasisTextureLoader: Add note in docs about iOS support.

docs/examples/loaders/BasisTextureLoader.html

@@ -60,6 +60,9 @@
 			desktop, Android, and iOS devices, and transcoded into DXT, ETC1, or
 			PVRTC1. Other output formats may be supported in the future.
 		</p>
+		<p>
+			Transcoding to PVRTC1 (for iOS) requires square power-of-two textures.
+		</p>
 		<p>
 			This loader relies on ES6 Promises and Web Assembly, which are not
 			supported in IE11.