Update README to clarify large heap

[louy2]

References:

• https://blog.openj9.org/2018/06/12/openj9-xl-heap/
• OpenJ9 Large Heap (noncompressedrefs) build adoptium/temurin-build#341
• Investigate making compressed references a runtime and not a compile time decision eclipse-openj9/openj9#643
• https://www.eclipse.org/openj9/docs/xcompressedrefs/

After making all changes to the cask:

• brew cask audit --download {{cask_file}} is error-free.
• brew cask style --fix {{cask_file}} reports no offenses.
• The commit message includes the cask’s name and version.

Additionally, if adding a new cask:

• Named the cask in a similar way to the existing casks.
• Added new cask to the README.md
• brew cask install {{cask_file}} worked successfully.
• brew cask uninstall {{cask_file}} worked successfully.
• Checked there are no [open pull requests] for the same cask.