JDK 16 RFR of JDK-8251549: Update docs on building for Git

[Joe Darcy]

Hello,

Ahead of the JDK switching to Git, please review the updates to the 
build docs:

    JDK-8251549: Update docs on building for Git
    http://cr.openjdk.java.net/~darcy/8251549.0/

This would be pushed shortly *after* the transition to Git.

Once the changes to md are agree to, I'll regenerate the companion html 
file.

Thanks,

-Joe