Add info about xcode-select in release instructions
Add info about xcode-select in release instructions

I had to use Xcode 12.2 release candidate to welcome the first machines
running on Apple M1 processors, as this version of Xcode is the first
one coming with macOS 11 SDK
Eitot/vienna-rss,Eitot/vienna-rss,barijaona/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss,ViennaRSS/vienna-rss,barijaona/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss