Update v5 readme

- New logo
- Add callout to v4-dev branch

README.md

@@ -1,6 +1,6 @@
 <p align="center">
   <a href="https://v5.getbootstrap.com/">
-    <img src="https://v5.getbootstrap.com/docs/5.0/assets/brand/bootstrap-solid.svg" alt="Bootstrap logo" width="72" height="72">
+    <img src="https://v5.getbootstrap.com/docs/5.0/assets/brand/bootstrap-logo-shadow.png" alt="Bootstrap logo" width="200" height="165">
   </a>
 </p>
 
@@ -22,6 +22,11 @@
 </p>
 
 
+## Bootstrap 4
+
+Our default branch is for development of our upcoming Bootstrap 5 release. Head to the [`v4-dev` branch](https://github.com/twbs/bootstrap/tree/v4-dev) to view the readme, documentation, and source code for Bootstrap 4.
+
+
 ## Table of contents
 
 - [Quick start](#quick-start)