[README] Mention ongoing migrations (#6685)

Co-authored-by: Sam Tran <samvantran@github.com>

README.md

@@ -5,6 +5,7 @@
 
 - [About](#about)
 - [Available Images](#available-images)
+- [Ongoing migrations](#ongoing-migrations)
 - [Image Definitions](#image-definitions)
 - [Image Releases](#image-releases)
 - [Software and Image Support](#software-and-image-support)
@@ -44,6 +45,14 @@ To build a VM machine from this repo's source, see the [instructions](docs/creat
 [macOS-10.15]: https://github.com/actions/runner-images/blob/main/images/macos/macos-10.15-Readme.md
 [self-hosted runners]: https://help.github.com/en/actions/hosting-your-own-runners
 
+## Ongoing migrations
+
+There are currently two migrations in progress:
+
+* `ubuntu-latest` is currently transitioning to `ubuntu-22.04`. During this time, you may experience some jobs running on either an ubuntu-20.04 or ubuntu-22.04 runner. You can specify `runs-on: ubuntu-20.04` in your workflow if you need the previous version. Details can be found in this [announcement](https://github.com/actions/runner-images/issues/6399).
+* `macos-latest` is currently transitioning to `macos-12`.  During this time, you may experience some jobs running on either a macos-11 or macos-12 runner. You can specify `runs-on: macos-11` in your workflow if you need the previous version. Details can be found in this [announcement](https://github.com/actions/runner-images/issues/6384).
+
+
 ## Image Definitions
 
 ### Beta