Caveat about old (3.6) pyenv python versions.

README.md

@@ -108,6 +108,11 @@ conan profile new --detect default
 python3 -m pip install pip==23.1.2
 ```
 
+Caveat: ensure that no old, say, 3.6 version of python is installed with
+pyenv as it will be picked up quite against your will, even if not
+activated, in a section of the build pertaining to pybind11. The python
+version constraints thus apply even to unactivated pyenv python version.
+
 # Build and run
 
 ## Rust library and REPL