chore: add note about default branch rename (#201)

Author: nrwiersma

README.md

@@ -21,6 +21,8 @@ Install with:
 go get github.com/hamba/avro/v2
 ```
 
+**Note:** This project has renamed the default branch from `master` to `main`. You will need to update your local environment.
+
 ## Usage
 
 ```go