Update build instructions in README.md
Update build instructions in README.md

This just updates the instructions again. Using this also to test CI.
alexperovich/corefxlab,benaadams/corefxlab,Vedin/corefxlab,ahsonkhan/corefxlab,whoisj/corefxlab,dotnet/corefxlab,ravimeda/corefxlab,whoisj/corefxlab,VSadov/corefxlab,adamsitnik/corefxlab,VSadov/corefxlab,stephentoub/corefxlab,stephentoub/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,ericstj/corefxlab,VSadov/corefxlab,Vedin/corefxlab,benaadams/corefxlab,Vedin/corefxlab,Vedin/corefxlab,weshaggard/corefxlab,stephentoub/corefxlab,Vedin/corefxlab,ericstj/corefxlab,ericstj/corefxlab,VSadov/corefxlab,adamsitnik/corefxlab,KrzysztofCwalina/corefxlab,stephentoub/corefxlab,hanblee/corefxlab,dotnet/corefxlab,Vedin/corefxlab,joshfree/corefxlab,stephentoub/corefxlab,tarekgh/corefxlab,ericstj/corefxlab,ahsonkhan/corefxlab,tarekgh/corefxlab,joshfree/corefxlab,KrzysztofCwalina/corefxlab,hanblee/corefxlab,ericstj/corefxlab,VSadov/corefxlab,VSadov/corefxlab