Enable webhooks on Circle

Summary:
Release Notes
=============

[INTERNAL][MINOR][CircleCI] - Send webhooks to Facebook.
Closes https://github.com/facebook/react-native/pull/19527

Differential Revision: D8228361

Pulled By: hramos

fbshipit-source-id: 5d7c0960392b9a860d3f665e2a316c18153879eb

.circleci/config.yml

@@ -672,3 +672,11 @@ workflows:
   #         filters: *filter-ignore-gh-pages
   #         requires:
   #           - checkout_code
+
+experimental:
+  notify:
+    webhooks:
+      - url: https://code.facebook.com/circle/webhook/
+    branches:
+      only:
+        - master