feat(README): add schema note

README.md

@@ -19,6 +19,8 @@ Creating and Contributing:
 - The easiest way to make your own, is to copy one of the existing 'brews as a template and use the main [5etools data](https://github.com/TheGiddyLimit/TheGiddyLimit.github.io/tree/master/data) as a reference.
 - Contributions are welcome. For the Git-literate, make a pull request. For everyone else, create an Issue and post a link to your file (PasteBin is fine), and I (or other maintainers, in future) can add it for you.
 
+A schema is available (`json.schema`) describing the layout of the and usage of the `_meta` property found in the various data files.
+
 Useful links:
 
  - https://5etools.com/demo.html (a demo of the renderer and JSON format)