Build/Test Tools: Add more comprehensive instructions to the readme for the...

Build/Test Tools: Add more comprehensive instructions to the readme for the local development environment.

See #50058
Built from https://develop.svn.wordpress.org/trunk@47743


git-svn-id: https://core.svn.wordpress.org/trunk@47519 1a063a9b-81f0-0310-95a4-ce76da25c4cd