Add note about tideways rather than xhprof to default.config.yml

Author: oxyc

default.config.yml

@@ -218,7 +218,7 @@ installed_extras:
  # - upload-progress
  - varnish
  # - xdebug
- # - xhprof
+ # - xhprof # use `tideways` if you're installing PHP 7+
 
 # Add any extra apt or yum packages you would like installed.
 extra_packages: