Use ws.css in the release notes.

Use the standard Wireshark style sheet for the release notes. Add a note
about updating them via themes.asciidoctor.org.

Change-Id: Ic6b66308cf1e1bea7b34a09963f92881d01eda8c
Reviewed-on: https://code.wireshark.org/review/25911
Reviewed-by: Anders Broman <a.broman58@gmail.com>

docbook/release-notes.asciidoc

@@ -1,4 +1,5 @@
 include::attributes.asciidoc[]
+:stylesheet: ws.css
 
 = Wireshark {wireshark-version} Release Notes
 // AsciiDoc quick reference: http://powerman.name/doc/asciidoc

docbook/ws.css

@@ -1,6 +1,9 @@
 /*
   CSS stylesheet for XHTML produced by DocBook XSL stylesheets.
   Derived from http://www.methods.co.nz/asciidoc/docbook-xsl.css
+
+  We might be able to get or make prettier CSS via
+  http://themes.asciidoctor.org/
 */
 
 span.strong {