git: openjdk/leyden: premain: Added instructions for running demos

[duke]

Changeset: 039c0219
Author:    iklam <ilam3388 at gmail.com>
Date:      2024-04-25 14:56:02 +0000
URL:       https://git.openjdk.org/leyden/commit/039c02197c2695b61e73c1418b7e56dace9d3ae3

Added instructions for running demos

! test/hotspot/jtreg/premain/README.md
! test/hotspot/jtreg/premain/lib/DemoSupport.gmk
! test/hotspot/jtreg/premain/spring-petclinic/Makefile