git: openjdk/jdk: 8280863: Update build README to reflect that MSYS2 is supported

Fri Jan 28 12:48:12 UTC 2022

Changeset: 6de90ad9
Author:    Magnus Ihse Bursie <mag at icus.se>
Committer: Magnus Ihse Bursie <ihse at openjdk.org>
Date:      2022-01-28 12:45:43 +0000
URL:       https://git.openjdk.java.net/jdk/commit/6de90ad9800b83c4a5f364c3645603fcb6828d6c

8280863: Update build README to reflect that MSYS2 is supported

Reviewed-by: ihse

! doc/building.html
! doc/building.md