Update config.md

Author: MichaelCurrin

cheatsheets/jekyll/config.md

@@ -3,6 +3,15 @@
 Jekyll docs reference - [Config options](https://jekyllrb.com/docs/configuration/options/). That covers both CLI and config file options together.
 
 
+## Load multiple configs
+
+Use a comma and no spaces to separate.
+
+```sh
+$ bundle exec jekyll build --trace \
+		--config _config.yml,_config_prod.yml
+```
+
 ## Null values
 
 ```yaml