Update P2 director documentation with new option -addJREIU

A new option is introduced by
eclipse-equinox/p2#484.

eclipse.platform.common/bundles/org.eclipse.platform.doc.isv/guide/p2_director.html

@@ -281,6 +281,10 @@ <h2>Arguments Description</h2>
 <dd>
 The SHA-256 'fingerprints' of unanchored certficates to trust as signers of artifacts. An empty value will reject all unanchored certificates.
 </dd>
+<dt>-addJREIU</dt>
+<dd>
+Include a default JRE installable unit as an extra IU. This IU satisfies JRE dependencies in the case that the content metadata does not otherwise provide an IU for that purpose.
+</dd>
 <dt>-help | -h | -?</dt>
 <dd>
 Prints this command line help information.