website: update GCN3 documentation

This commit updates the GCN3 GPU documentation to make it clear
that the GPU model is currently only supported on the develop
branch, not stable.

Change-Id: Iab4d07f97edeff37384d2a0adbfb8a402df90a56
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/42803
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/documentation/general_docs/gpu_models/gcn3.md b/_pages/documentation/general_docs/gpu_models/gcn3.md
index f7a2811..fe006a7 100644
--- a/_pages/documentation/general_docs/gpu_models/gcn3.md
+++ b/_pages/documentation/general_docs/gpu_models/gcn3.md
@@ -18,6 +18,10 @@
 
 ## **Using the model**
 
+Currently, the GCN3 GPU model in gem5 is only supported on the develop branch.
+Thus, you should use the develop branch when running the GCN3 model.
+We expect this support to be integrated into the gem5-v21.0 release.
+
 The [gem5 repository](https://gem5.goooglesource.com/public/gem5) comes with a dockerfile located in `util/dockerfiles/gcn-gpu/`. This dockerfile contains the drivers and libraries needed to run the GPU model
 
 The [gem5-resources repository](https://gem5.googlesource.com/public/gem5-resources/) also comes with a sample application (square) that can be used to verify that the model runs correctly.
@@ -82,7 +86,7 @@
 * [MIOpen](https://github.com/ROCmSoftwarePlatform/MIOpen/)
 * [rocm-cmake](https://github.com/RadeonOpenCompute/rocm-cmake/)
 
-For information about installing these components locally, the commands in the GCN3 dockerfile (`util/dockerfiles/gcn-gpu/`) can be followed on an ubuntu 16 machine.
+For information about installing these components locally, the commands in the GCN3 dockerfile (`util/dockerfiles/gcn-gpu/`) can be followed on an Ubuntu 16 machine.
 
 ## **Documentation and Tutorials**