Adds note to README about testing v4

Author: crcastle

README.md

@@ -7,3 +7,7 @@ Fork this repo and click the button below to deploy.
 [![Deploy to Render](https://render.com/images/deploy-to-render-button.svg)](https://render.com/deploy)
 
 See the guide at https://render.com/docs/deploy-strapi for more information.
+
+## Test Strapi v4
+
+We plan to update this repository to v4 of Strapi shortly after [StrapiConf 2022](https://conf.strapi.io), however you may test out v4 on Render now by selecting the [`crc/upgrade-to-v4`](https://github.com/render-examples/strapi-postgres/pull/9) branch during deploy.