Add readme notification

Please make all new spec updates in https://github.com/w3c/aria/tree/main/html-aam

README.md

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 # HTML-AAM
+
+*****
+ PLEASE NOTE
+
+ The files for this spec are now being updated in the ARIA repository: https://github.com/w3c/aria/tree/main/html-aam
+
+ All new updates need to be made in those files.
+
+ Please do not make new PRs against this repository. 
+*****
+
+
 HTML Accessibility API Mappings (html-aam) developed by the [Accessible Rich Internet Applications Working Group](https://www.w3.org/WAI/ARIA/) (previously the [Web Applications Working Group](https://www.w3.org/2019/webapps/)). It is the basis of the [Editor's Draft version of the specification](https://w3c.github.io/html-aam/).  You can also compare against [the most recent published version](https://www.w3.org/TR/html-aam-1.0/).
 
 This specification was part of the [ARIA suite](https://www.w3.org/WAI/ARIA/deliverables). General information about editing the specification is in the [main ARIA repository readme](https://github.com/w3c/aria/).