Update README to clarify large heap

References: 
- https://blog.openj9.org/2018/06/12/openj9-xl-heap/
- adoptium/temurin-build#341
- eclipse-openj9/openj9#643
- https://www.eclipse.org/openj9/docs/xcompressedrefs/

README.md

@@ -16,12 +16,17 @@ brew cask install <version>
 |--|--|--|
 | OpenJDK8 with Hotspot JVM | `adoptopenjdk8` | `adoptopenjdk8-jre` |
 | OpenJDK8 with OpenJ9 JVM | `adoptopenjdk8-openj9` | `adoptopenjdk8-openj9-jre` |
-| OpenJDK8 with OpenJ9 JVM, large heap | `adoptopenjdk8-openj9-large` | `adoptopenjdk8-openj9-jre-large` |
+| OpenJDK8 with OpenJ9 JVM, large heap* | `adoptopenjdk8-openj9-large` | `adoptopenjdk8-openj9-jre-large` |
 | OpenJDK9 with Hotspot JVM | `adoptopenjdk9` | n/a |
 | OpenJDK10 with Hotspot JVM | `adoptopenjdk10` | n/a |
 | OpenJDK11 with Hotspot JVM | `adoptopenjdk11` | `adoptopenjdk11-jre` |
 | OpenJDK11 with OpenJ9 JVM | `adoptopenjdk11-openj9` | `adoptopenjdk11-openj9-jre` |
-| OpenJDK11 with OpenJ9 JVM, large heap | `adoptopenjdk11-openj9-large` | `adoptopenjdk11-openj9-jre-large` |
+| OpenJDK11 with OpenJ9 JVM, large heap* | `adoptopenjdk11-openj9-large` | `adoptopenjdk11-openj9-jre-large` |
 | OpenJDK12 with Hotspot JVM | `adoptopenjdk12` | `adoptopenjdk12-jre` |
 | OpenJDK12 with OpenJ9 JVM | `adoptopenjdk12-openj9` | `adoptopenjdk12-openj9-jre` |
-| OpenJDK12 with OpenJ9 JVM, large heap | `adoptopenjdk12-openj9-large` | `adoptopenjdk12-openj9-jre-large` |
+| OpenJDK12 with OpenJ9 JVM, large heap* | `adoptopenjdk12-openj9-large` | `adoptopenjdk12-openj9-jre-large` |
+
+\* Large heap: Use this if you need heap memory > 57GB, or > 25GB on Linux. It enables `-Xnocompressedref` ([doc][docnocref]). Use it along with `-Xmx` ([doc][docmx]) which sets the max heap size.
+
+[docnocref]: https://www.eclipse.org/openj9/docs/xcompressedrefs/
+[docmx]: https://www.eclipse.org/openj9/docs/xms/