Add build status explanation in master list

README.md

@@ -4,6 +4,8 @@ A collective list of free JSON APIs for use in web development.
 
 For information on contributing to this project, please see the [contributing guide](CONTRIBUTING.md).
 
+Please note a passing build status indicates all listed APIs are available since the last update. A failing build status indicates that 1 or more services may be unavailable at the moment.
+
 ## Index
 
 * [Animals](#animals)