Update .rules to prompt for self-review before submitting PR (#62945)

Release Notes:

- N/A

script/danger/dangerfile.ts

@@ -1,4 +1,4 @@
-import { danger, message, warn, fail } from "danger";
+import { danger, message, warn, fail, schedule } from "danger";
 const { prHygiene } = require("danger-plugin-pr-hygiene");
 
 prHygiene({
@@ -61,6 +61,16 @@ if (includesIssueUrl) {
   );
 }
 
+const readmeModified = danger.git.modified_files.includes("README.md") || danger.git.created_files.includes("README.md");
+if (readmeModified) {
+  schedule(async () => {
+    const readmeDiff = await danger.git.diffForFile("README.md");
+    if (readmeDiff && readmeDiff.added.includes("Remove this line to confirm you've reviewed this PR before submitting.")) {
+      fail("Please self-review your PR before submitting — README.md contains a line that asks to be removed which you should have spotted.");
+    }
+  });
+}
+
 const SCHEMA_CHANGE_ATTESTATION =
   "The corresponding database schema migration has been created in the Cloud repo and applied to the production database.";