Update README.md

Author: alivest

README.md

@@ -9,3 +9,7 @@ Table of contents: [index.md](/doc_source/index.md)
 The documentation is made available under the Creative Commons Attribution-ShareAlike 4.0 International License. See the [LICENSE](./LICENSE) file.
 
 The sample code within this documentation is made available under a modified MIT license. See the [LICENSE-SAMPLECODE](./LICENSE-SAMPLECODE) file.
+
+## Default branch
+The default branch for this repo has changed to `main`. 
+If you have cloned the previous default branch, please update your local repo to use the `main` branch.