docs(no-large-snapshot): mention --ext snap

Author: G-Rath

docs/rules/no-large-snapshots.md

@@ -24,6 +24,9 @@ module.exports = {
 };
 ```
 
+In order to check external snapshots, you must also have `eslint` check files
+with the `.snap` extension by passing `--ext snap`.
+
 ## Rule details
 
 This rule looks at all Jest inline and external snapshots (files with `.snap`