Add --host advice to docc preview instructions

Because `localhost` only allows connections on the loopback interface,
anybody who’s trying to run `docc preview` on a remote server and
connect to it with their local browser will need to use `--host 0.0.0.0`
or similar.

CONTRIBUTING.md

@@ -288,7 +288,10 @@ Monitoring ~/Developer/swift-docc/Sources/SwiftDocC/SwiftDocC.docc for changes..
 ```
 
 And if you navigate to <http://localhost:8080/documentation/swiftdocc> you'll see
-the rendered documentation for `SwiftDocC`.
+the rendered documentation for `SwiftDocC`. If your browser isn't running on
+the same computer as Swift-DocC, you may need to provide the server's public
+host name or IP address (or just `0.0.0.0`) with the `--host` option to allow
+the browser to connect.
 
 ### Using Docker to Test Swift-DocC for Linux