Add release banner script to readme (#16461)

* Add release banner script to readme

* Update script/README.md

Author: myarb

script/README.md

@@ -368,6 +368,13 @@ An automated test checks for discrepancies between filenames and [autogenerated
 ---
 
 
+### [`release-banner.js`](release-banner.js)
+
+This script creates or removes a release candidate banner for a specified version.
+
+
+---
+
 ### [`remove-deprecated-enterprise-version-markup.js`](remove-deprecated-enterprise-version-markup.js)
 
 Run this script after an Enterprise deprecation to remove Liquid statements and frontmatter that contain the deprecated Enterprise version. See the Enterprise deprecation issue template for instructions.
@@ -487,4 +494,3 @@ Run this script to: [upload individual files to S3](https://github.com/github/pr
 
 ---
 
-