Clarify compiler support

Fixes: #1967

README.md

@@ -244,6 +244,8 @@ Unless otherwise specified, CCCL supports all the same host compilers as the CUD
 - [Linux](https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html#host-compiler-support-policy)
 - [Windows](https://docs.nvidia.com/cuda/cuda-installation-guide-microsoft-windows/index.html#system-requirements)
 
+However, compilers not tested by our CI are unsupported. You can find the list of our CI containers [here](.devcontainer).
+
 ### C++ Dialects
 - C++11 (Deprecated in Thrust/CUB, to be removed in next major version)
 - C++14 (Deprecated in Thrust/CUB, to be removed in next major version)