sdk: note that an x86-64 host is required
sdk: note that an x86-64 host is required

This fixes issue #7. Thanks to Jeff Hackert for the report.

bq-xiao/docs,joshix/docs,jonboulle/docs,robszumski/docs,iankronquist/docs,hkjn/docs-1,endocode/coreos-docs,ionave/docs,mischief/coreos-docs,wli6/docs,marineam/coreos-docs,mjg59/docs,michelleN/docs,ksmaheshkumar/docs-2,coreos/docs,atxwebs/docs,jdloft/docs,skynet/docs