Clarify use of this tap with existing Linuxbrew
Clarify use of this tap with existing Linuxbrew

Issue #12 highlighted that this tap can't yet be used directly with an existing Linuxbrew install
due to CERN ROOT incompatibilities with Linuxbrew's gcc/glibc install.

Note this issue and describe install procedure for SuperNEMO's fork of Linuxbrew.
SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael,SuperNEMO-DBD/homebrew-cadfael,xgarrido/homebrew-cadfael