chore: migration warnings (#3538)

Author: Foxandxss

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,7 @@
+**Please do not add issues or pull requests to this repo.** 
+We are no longer making changes to documentation in this repository.
+We will no longer process new issues or PRs and we will close them automatically.
+
+**Please post new [issues](https://github.com/angular/angular/issues) and [pull requests](https://github.com/angular/angular/pulls) to the content folder in [https://github.com/angular/angular/tree/master/aio/content](https://github.com/angular/angular/tree/master/aio/content)**.
+
+Be sure to prefix your issue/PR title with "**docs(aio):**"

.github/PULL_REQUEST_TEMPLATE.MD

@@ -0,0 +1,7 @@
+**Please do not add issues or pull requests to this repo.**
+We are no longer making changes to documentation in this repository.
+We will no longer process new issues or PRs and we will close them automatically.
+
+**Please post new [issues](https://github.com/angular/angular/issues) and [pull requests](https://github.com/angular/angular/pulls) to the content folder in [https://github.com/angular/angular/tree/master/aio/content](https://github.com/angular/angular/tree/master/aio/content)**.
+
+Be sure to prefix your issue/PR title with "**docs(aio):**"

README.md

@@ -13,6 +13,18 @@ Be sure to prefix your issue/PR title with "**docs(aio):**"
 # Angular.io
 [![Build Status][travis-badge]][travis-badge-url]
 
+# This angular.io documentation repository is closed
+
+**Please do not add issues or pull requests to this repo.** 
+We are no longer making changes to documentation in this repository.
+We will no longer process new issues or PRs and we will close them automatically.
+
+**Please post new [issues](https://github.com/angular/angular/issues) and [pull requests](https://github.com/angular/angular/pulls) to the content folder in [https://github.com/angular/angular/tree/master/aio/content](https://github.com/angular/angular/tree/master/aio/content)**.
+
+Be sure to prefix your issue/PR title with "**docs(aio):**"
+
+---
+
 Angular.io is site for Angular **documentation** . 
 
 This site also includes links to other helpful angular resources including