Replace 'main' with 'default' and format

Author: GMNGeoffrey

README.md

@@ -1,7 +1,10 @@
 # IREE llvm-project fork
 
-This repository contains branches for various forks of LLVM in support of the [IREE](https://github.com/google/iree) project. This main branch is protected and preserves this message.
+This repository contains branches for various forks of LLVM in support
+of the [IREE](https://github.com/google/iree) project. This default
+branch is protected and preserves this message.
 
-Contact the [IREE Team](mailto:[email protected]) for questions about this fork.
+Contact the [IREE Team](mailto:[email protected]) for
+questions about this fork.