Update README.md to discourage use

Author: nainar

README.md

@@ -1,14 +1,16 @@
-#What is this?
+# PLEASE NOTE THAT THIS REPO IS OUT OF DATE. PLEASE DO NOT USE!
+
+# What is this?
 This is a proof-of-concept polyfill for the proposed [Typed CSSOM API](https://drafts.css-houdini.org/css-typed-om-1/).
 
-#Using this Polyfill
+# Using this Polyfill
 To use the code, download this repository and put
 ```
 <script src='typed-om/typed-om.js'></script>
 ```
 in your HTML file.
 
-#Testing this Polyfill
+# Testing this Polyfill
 The tests use the [Mocha framework](https://mochajs.org/) with [Chai asserts](http://chaijs.com/api/assert/). Other Chai tools can be pulled in if needed.
 
 To run the tests do