Docs: Document the deprecated meta property (#7367)

docs/developer-guide/working-with-rules.md

@@ -61,6 +61,8 @@ The source file for a rule exports an object with the following properties.
 
 * `schema` (array) specifies the [options](#options-schemas) so ESLint can prevent invalid [rule configurations](../user-guide/configuring#configuring-rules)
 
+* `deprecated` (boolean) indicates whether the rule has been deprecated.  You may omit the `deprecated` property if the rule has not been deprecated.
+
 `create` (function) returns an object with methods that ESLint calls to "visit" nodes while traversing the abstract syntax tree (AST as defined by [ESTree](https://github.com/estree/estree)) of JavaScript code:
 
 * if a key is a node type, ESLint calls that **visitor** function while going **down** the tree