Build/Test Tools: Add information regarding the default credentials for the...

Build/Test Tools: Add information regarding the default credentials for the local development environment to the project readme.

Props bookdude13

Fixes #49273
Built from https://develop.svn.wordpress.org/trunk@47744


git-svn-id: https://core.svn.wordpress.org/trunk@47520 1a063a9b-81f0-0310-95a4-ce76da25c4cd