INSTALL.md: Mention -j and other wibbles
INSTALL.md: Mention -j and other wibbles

(cherry picked from commit b0641ad9009df1e3f4eae4274668840ff6e46732)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc