Update building documentation for osx/macOS (#12366)
Update building documentation for osx/macOS (#12366)

* Update osx-instructions.md

add 10.12 Sierra as minimal needed version and add some more notes for pkg-config add reference to clean.sh

pgavlin/coreclr,botaberg/coreclr,dpodder/coreclr,YongseopKim/coreclr,gkhanna79/coreclr,poizan42/coreclr,AlexGhiondea/coreclr,poizan42/coreclr,AlexGhiondea/coreclr,gkhanna79/coreclr,cydhaselton/coreclr,botaberg/coreclr,rartemev/coreclr,ragmani/coreclr,dpodder/coreclr,botaberg/coreclr,wateret/coreclr,cydhaselton/coreclr,YongseopKim/coreclr,hseok-oh/coreclr,ragmani/coreclr,wtgodbe/coreclr,wateret/coreclr,JonHanna/coreclr,gkhanna79/coreclr,cshung/coreclr,cydhaselton/coreclr,tijoytom/coreclr,ragmani/coreclr,rartemev/coreclr,mmitche/coreclr,ruben-ayrapetyan/coreclr,parjong/coreclr,pgavlin/coreclr,gkhanna79/coreclr,cshung/coreclr,ruben-ayrapetyan/coreclr,mskvortsov/coreclr,rartemev/coreclr,cydhaselton/coreclr,kyulee1/coreclr,parjong/coreclr,yizhang82/coreclr,tijoytom/coreclr,YongseopKim/coreclr,mmitche/coreclr,yizhang82/coreclr,JosephTremoulet/coreclr,wtgodbe/coreclr,ragmani/coreclr,mmitche/coreclr,botaberg/coreclr,kyulee1/coreclr,russellhadley/coreclr,dpodder/coreclr,parjong/coreclr,botaberg/coreclr,cydhaselton/coreclr,dpodder/coreclr,yizhang82/coreclr,gkhanna79/coreclr,gkhanna79/coreclr,pgavlin/coreclr,parjong/coreclr,kyulee1/coreclr,pgavlin/coreclr,ruben-ayrapetyan/coreclr,tijoytom/coreclr,botaberg/coreclr,krk/coreclr,poizan42/coreclr,YongseopKim/coreclr,russellhadley/coreclr,mskvortsov/coreclr,hseok-oh/coreclr,mmitche/coreclr,tijoytom/coreclr,poizan42/coreclr,tijoytom/coreclr,JosephTremoulet/coreclr,cshung/coreclr,mskvortsov/coreclr,AlexGhiondea/coreclr,krk/coreclr,russellhadley/coreclr,JosephTremoulet/coreclr,wateret/coreclr,poizan42/coreclr,kyulee1/coreclr,JonHanna/coreclr,wtgodbe/coreclr,krk/coreclr,mskvortsov/coreclr,dpodder/coreclr,kyulee1/coreclr,pgavlin/coreclr,JonHanna/coreclr,cshung/coreclr,AlexGhiondea/coreclr,rartemev/coreclr,cydhaselton/coreclr,tijoytom/coreclr,krk/coreclr,JosephTremoulet/coreclr,rartemev/coreclr,dpodder/coreclr,JonHanna/coreclr,mskvortsov/coreclr,russellhadley/coreclr,wateret/coreclr,mmitche/coreclr,parjong/coreclr,YongseopKim/coreclr,wateret/coreclr,hseok-oh/coreclr,cshung/coreclr,yizhang82/coreclr,hseok-oh/coreclr,russellhadley/coreclr,ruben-ayrapetyan/coreclr,wtgodbe/coreclr,krk/coreclr,JonHanna/coreclr,wtgodbe/coreclr,JosephTremoulet/coreclr,rartemev/coreclr,poizan42/coreclr,kyulee1/coreclr,russellhadley/coreclr,krk/coreclr,mmitche/coreclr,ragmani/coreclr,JosephTremoulet/coreclr,hseok-oh/coreclr,AlexGhiondea/coreclr,mskvortsov/coreclr,wateret/coreclr,cshung/coreclr,YongseopKim/coreclr,yizhang82/coreclr,JonHanna/coreclr,pgavlin/coreclr,ragmani/coreclr,yizhang82/coreclr,ruben-ayrapetyan/coreclr,hseok-oh/coreclr,wtgodbe/coreclr,AlexGhiondea/coreclr,ruben-ayrapetyan/coreclr,parjong/coreclr