 Updating the building and testing steps within the README of the repo. (#2305)
 Updating the building and testing steps within the README of the repo. (#2305)

* Updating the building and testing steps within the README of the repo.

* Add more details and tweak some steps.

* Add back link to install the latest .NET CLI.

ahsonkhan/corefxlab,KrzysztofCwalina/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,joshfree/corefxlab,whoisj/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,ericstj/corefxlab,tarekgh/corefxlab,stephentoub/corefxlab,benaadams/corefxlab,adamsitnik/corefxlab,whoisj/corefxlab,adamsitnik/corefxlab,stephentoub/corefxlab,KrzysztofCwalina/corefxlab,benaadams/corefxlab,ahsonkhan/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,stephentoub/corefxlab,ericstj/corefxlab,tarekgh/corefxlab,joshfree/corefxlab,dotnet/corefxlab,dotnet/corefxlab,ericstj/corefxlab