Add the first changelog entry

(Adding the changelog itself.)

.gitattributes

@@ -1 +1,8 @@
 lib/*.js linguist-generated=true
+
+# Reduce incidence of needless merge conflicts on CHANGELOG.md
+# The man page at
+# https://mirrors.edge.kernel.org/pub/software/scm/git/docs/gitattributes.html
+# suggests that this might interleave lines arbitrarily, but empirically
+# it keeps added chunks contiguous
+CHANGELOG.md                merge=union

CHANGELOG.md

@@ -1,3 +1,5 @@
 # CodeQL Action and CodeQL Runner Changelog
 
 ## [UNRELEASED]
+
+- Add this changelog file. [#507](https://github.com/github/codeql-action/pull/507)

README.md

@@ -2,6 +2,8 @@
 
 This action runs GitHub's industry-leading static analysis engine, CodeQL, against a repository's source code to find security vulnerabilities. It then automatically uploads the results to GitHub so they can be displayed in the repository's security tab. CodeQL runs an extensible set of [queries](https://github.com/github/codeql), which have been developed by the community and the [GitHub Security Lab](https://securitylab.github.com/) to find common vulnerabilities in your code.
 
+For a list of recent changes, see the CodeQL Action's [changelog](CHANGELOG.md).
+
 ## License
 
 This project is released under the [MIT License](LICENSE).