Update README with Firebase ID token verification

Added Firebase ID token verification example to README.

Signed-off-by: ranette832-ux <ranette832@gmail.com>

README.md

@@ -1,4 +1,12 @@
-# Helm
+# Helmtry {
+  FirebaseAuth.getInstance().verifyIdToken(idToken, true);
+} catch (FirebaseAuthException ex) {
+  if (ex.getAuthErrorCode() == AuthErrorCode.REVOKED_ID_TOKEN) {
+    System.err.println("ID token has been revoked");
+  } else {
+    System.err.println("ID token is invalid");
+  }
+}
 
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