Update changelog and version after v1.0.1

CHANGELOG.md

@@ -1,5 +1,7 @@
 # CodeQL Action and CodeQL Runner Changelog
 
+## [UNRELEASED]
+
 ## 1.0.1 - 07 Jun 2021
 
 - Pass the `--sarif-group-rules-by-pack` argument to CodeQL CLI invocations that generate SARIF. This means the SARIF rule object for each query will now be found underneath its corresponding query pack in `runs[].tool.extensions`. [#546](https://github.com/github/codeql-action/pull/546)