Update doc/ci/yaml/README.md

doc/ci/yaml/README.md

@@ -1631,6 +1631,10 @@ rspec:
     - bundle exec rspec
 ```
 
+NOTE: **Note:**
+`include` requires the external YAML files to have the extensions `.yml` or `.yaml`. 
+The external file will not be included if the extension is missing.
+
 You can define it either as a single string, or, in case you want to include
 more than one files, an array of different values . The following examples
 are both valid cases: