Deprecate contentBase with a URL or port

This was originally introduced before the `proxy` option was a thing.
Now we have the `proxy` option, it is better to use that instead
since it is way more powerful and can do the same.

lib/Server.js

@@ -239,6 +239,8 @@ function Server(compiler, options) {
 					app.get("*", express.static(item));
 				});
 			} else if(/^(https?:)?\/\//.test(contentBase)) {
+				console.log("Using a URL as contentBase is deprecated and will be removed in the next major version. Please use the proxy option instead.");
+				console.log('proxy: {\n\t"*": "<your current contentBase configuration>"\n}'); // eslint-disable-line quotes
 				// Redirect every request to contentBase
 				app.get("*", function(req, res) {
 					res.writeHead(302, {
@@ -247,6 +249,8 @@ function Server(compiler, options) {
 					res.end();
 				});
 			} else if(typeof contentBase === "number") {
+				console.log("Using a number as contentBase is deprecated and will be removed in the next major version. Please use the proxy option instead.");
+				console.log('proxy: {\n\t"*": "//localhost:<your current contentBase configuration>"\n}'); // eslint-disable-line quotes
 				// Redirect every request to the port contentBase
 				app.get("*", function(req, res) {
 					res.writeHead(302, {