[jdk11u notice] Labels documented

[Lindenmaier, Goetz]

Hi,

we decided to document some labels we use for jdk11u issues that
we do not backport, see [1].

Also, we decided not to use openjdk-na any more. When we started
this, jdk11u was the only updates project run by the open community.
Now, as there are many, this label is ambiguous.  Instead, we
now use jdk11u-na.

The other labels documented are
  jdk11u-jvmci-defer
  jdk11u-solaris-sparc-defer
  jdk11u-nsk-8209611-defer

Best regards,
  Goetz.

[1] https://wiki.openjdk.java.net/display/JDKUpdates/JDK11u Section "Common jdk11u JBS labels" at the bottom.