Fix up README.md
Fix up README.md

miss a line "cd build" in section "How to build"

freedesktop-unofficial-mirror/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,ignatenkobrain/beignet,wdv4758h/beignet,zhenyw/beignet,zhenyw/beignet,ignatenkobrain/beignet,wdv4758h/beignet,wdv4758h/beignet,ignatenkobrain/beignet,zhenyw/beignet,ignatenkobrain/beignet,freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,ignatenkobrain/beignet,zhenyw/beignet,freedesktop-unofficial-mirror/beignet