added description of PKGCONFIG_INSTALL_PREFIX to INSTALL instructions
added description of PKGCONFIG_INSTALL_PREFIX to INSTALL instructions

KDE/qca,Bjoe/qca,KDE/qca,Bjoe/qca,Bjoe/qca,sladage/qca,highfidelity/qca,sladage/qca,karolherbst/qca,Esf-Software/qca,JoshuaKolden/qca,oktavarium/qca,KDE/qca,Bjoe/qca,sladage/qca,Esf-Software/qca,karolherbst/qca,oktavarium/qca,oktavarium/qca,highfidelity/qca,KDE/qca,oktavarium/qca,sladage/qca,JoshuaKolden/qca,karolherbst/qca,Esf-Software/qca,karolherbst/qca,highfidelity/qca,highfidelity/qca,Esf-Software/qca,JoshuaKolden/qca,JoshuaKolden/qca