added warning to notify SingleFile is hooking the FontFace constructor

lib/single-file/font-face-proxy.js

@@ -50,6 +50,7 @@ this.fontFaceProxy = this.fontFaceProxy || (() => {
 		const FontFace = window.FontFace;
 		window.__defineGetter__("FontFace", () => new Proxy(FontFace, {
 			construct(FontFace, argumentsList) {
+				console.warn("SingleFile is hooking The FontFace constructor to get font URLs."); // eslint-disable-line no-console
 				const detail = {};
 				detail["font-family"] = argumentsList[0];
 				detail.src = argumentsList[1];