Update README to include MRVA

Author: charisk

README.md

@@ -15,6 +15,7 @@ To see what has changed in the last few versions of the extension, see the [Chan
 * Shows the flow of data through the results of path queries, which is essential for triaging security results.
 * Provides an easy way to run queries from the large, open source repository of [CodeQL security queries](https://github.com/github/codeql).
 * Adds IntelliSense to support you writing and editing your own CodeQL query and library files.
+* Supports you running CodeQL queries against thousands of repositories on GitHub using multi-repository variant analysis.
 
 ## Project goals and scope