Include directions in examples/README.md to have compatible versions of bb-runtimes and compiler

[filip-gajowniczek-adacore]

When following the directions in https://github.com/AdaCore/Ada_Drivers_Library/blob/master/examples/README.md

The following command will clone the bb-runtimes repository under the hood:
python ./scripts/install_dependencies.py

When run with arm-elf-linux 21.1 Ada compiler intalled, the bb-runtimes that is checked out by default is incompatible with the version of the installed compiler. Manually checking out branch 21.1 in the bb-runtimes repository that was created by the python script resolved the issue.