Merge pull request #1166 from spk/fix_example_env_file

Docs: fix env_file example

docs/yml.md

@@ -173,8 +173,12 @@ env_file:
   - /opt/secrets.env
 ```
 
+Compose expects each line in an env file to be in `VAR=VAL` format. Lines
+beginning with `#` (i.e. comments) are ignored, as are blank lines.
+
 ```
-RACK_ENV: development
+# Set Rails/Rack environment
+RACK_ENV=development
 ```
 
 ### extends