Add documentation for integrating shellcheck with Buildkite

Buildkite.md

@@ -0,0 +1,14 @@
+Buildkite provides [a plugin dedicated to run `shellcheck`](https://github.com/buildkite-plugins/shellcheck-buildkite-plugin).
+
+Simply add this step to your Buildkite pipeline (`.buildkite/pipeline.yml`) to use it:
+
+```yaml
+steps:
+  - label: ":shell: Shellcheck"
+    plugins:
+      shellcheck#v1.1.2:
+        # List the files you want to lint
+        files: ["**/*.sh"]
+        # additional options to pass to shellcheck (optional)
+        options: ["-x", "-P", "SCRIPTDIR"]
+```