docs: added additional note about httpModule option - fixes #1485

Author: shakyShane

lib/default-config.js

@@ -164,6 +164,8 @@ module.exports = {
 
     /**
      * Override http module to allow using 3rd party server modules (such as http2)
+     * *Note*: these modules are not included in the Browsersync package - you need
+     * to 'npm install' any that you'd like to use.
      * @property httpModule
      * @type string
      * @default undefined