docs: Provide syntax for specifying configuration (#778)

Author: monkbroc

docs/CONFIG.md

@@ -9,6 +9,11 @@ Configuration - a `documentation.yml` file - will accomplish two goals:
 * Organization: you can put top level documentation in order of importance
 * Narration: you can add narrative sections - plain English writing - in
   between autogenerated API documentation.
+  
+Specify the configuration file with the `--config` command-line option.
+```sh
+$ documentation build --config documentation.yml ...
+```
 
 Here's how `documentation.yml` works: