Update docs on C++ Standard
Update docs on C++ Standard

All Cadfael formulae now install with C++11 as the only allowed
standard. Update notes to record this and how to cowork with Formulae
from other taps to ensure binary compatibility.

SuperNEMO-DBD/homebrew-cadfael,SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael,xgarrido/homebrew-cadfael