add installation instructions for Gentoo
add installation instructions for Gentoo

jongillham/btcd,dan-da/btcd,ltcsuite/ltcd,ltcsuite/ltcd,Stash-Crypto/btcd,jrick/dcrd,soapboxsys/ombfullnode,jrick/dcrd,jrick/btcd,jongillham/btcd,jrick/btcd,dajohi/btcd,jcmincke/dcrd,Stash-Crypto/btcd,jcmincke/dcrd,dajohi/btcd,stevenroose/btcd,Roasbeef/btcd,tb00/btcd,adiabat/btcd,dan-da/btcd,wallclockbuilder/btcd,BitGo/prova,hectorj/btcd,wallclockbuilder/btcd,btcsuite/btcd,aakselrod/btcd,ppcsuite/ppcd,decred/dcrd,tb00/btcd,adiabat/btcd,cqswxj2013/btcd,aakselrod/btcd,jcmincke/dcrd,btcsuite/btcd,decred/dcrd,davecgh/btcd,soapboxsys/ombfullnode,ppcsuite/ppcd,cqswxj2013/btcd,jrick/btcd,runeaune/btcd,dajohi/btcd,hectorj/btcd,BitGo/prova,runeaune/btcd,stevenroose/btcd,davecgh/btcd,Roasbeef/btcd