Added release structure doc and updated master README

README.md

@@ -1,5 +1,16 @@
 # CBT - The Ceph Benchmarking Tool
 
+---
+> ⚠️ **IMPORTANT NOTICE**
+>
+> This current branch (`master`) is **frozen**. Please refer to
+> the [**`main` branch**](https://github.com/ceph/cbt/tree/main)
+> which contains the latest features and code for CBT.
+> 
+> Also see information about the new [release structure](docs/ReleaseStructure.md) for CBT
+> following the release of Ceph v21.x (Umbrella).
+---
+
 ## INTRODUCTION
 
 CBT is a testing harness written in python that can automate a variety of tasks

docs/ReleaseStructure.md

@@ -0,0 +1,22 @@
+# CBT Release Structure
+
+## Proposal
+
+From Ceph v21.x (Umbrella) onwards, there will be specific 
+branches of CBT that tie to each major Ceph release.
+Depnding on the version of Ceph which teuthology is running 
+against, the corresponding branch of CBT will be used.
+
+## Branches
+- The `main` branch is the new default branch for ongoing CBT 
+development. It is also used for standalone CBT deployments.
+
+- The `master` branch will be retained for backwards compatibility
+with older Ceph versions. This branch is frozen and will receive 
+no further updates.
+
+- For each new Ceph release, a new CBT branch for that release 
+will be created. These branches are exclusively for running
+teuthology for a specific Ceph release.
+
+![CBT Release diagram](./cbt_release_structure.png)