Add SKIP_COMPOSER to docs

Author: auchri

docs/config_flags.md

@@ -24,3 +24,4 @@ The following flags are a list of all the currently supported options that can b
  - **REMOVE_FILES** : Use REMOVE_FILES=0 to prevent the script from clearing out /var/www/html (useful for working with local files)
  - **APPLICATION_ENV** : set this to development to prevent composer deleteing local dev dependancies
  - **SKIP_CHOWN** : set to 1 to avoid running chown -Rf on /var/www/html
+ - **SKIP_COMPOSER** : set to 1 to avoid installing composer