Note when javavendors_*.xml <updated> should be updated

Change-Id: I387e3c798feae864c046a9274c903b9aa679399b
Reviewed-on: https://gerrit.libreoffice.org/69731
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

jvmfwk/README

@@ -2,3 +2,14 @@ Wrappers so you can use all the Java Runtime Environments with their slightly in
 
 Used to use an over-engineered "plugin" mechanism although there was only one
 "plugin", called "sunmajor", that handles all possible JREs.
+
+IMPORTANT:  The <updated> element in vmfwk/distributions/OpenOfficeorg/javavendors_*.xml files
+should only be updated for incompatible changes, not for compatible ones.  As stated in the commit
+message of <https://gerrit.libreoffice.org/#/c/69730/> "javavendors_*.xml <updated> should not have
+been updated...":  "Changing <updated> causes jfw_startVM and jfw_getSelectedJRE (both
+jvmfwk/source/framework.cxx) to fail with JFW_E_INVALID_SETTINGS, which in turn causes functionality
+that requires a JVM to issue a GUI error dialog stating that the user must select a new JRE in the
+Options dialog.  While that behavior makes sense if a JRE was selected that would no longer be
+supported by the modified javavendors_*.xml, it is just annoying if an already selected JRE is still
+supported.  And a compatible change to javavendors_*.xml implies that an already selected JRE will
+still be supported."