add a note for public beta

Author: hectorsector

README.md

@@ -8,4 +8,8 @@ Register for this course to get the starter files needed to build your Learning
 - create responses
 - publish a course to GitHub Learning Lab
 
-**Note:** In order to publish your final course, you'll need to transfer it to your designated Learning Lab organization.
+> Note: Creating courses on Learning Lab is currently available in public beta. If you're not a member of the public beta, you will be unable to publish the course you create with this course.
+>
+> Features and requirements may change at any time during this period. You can request to join the public beta by [contacting us](https://lab.github.com/organizations#form).
+>
+> In order to publish your final course, you'll need to transfer it to your designated Learning Lab organization.