add patchstack vdp link

Author: dannyvankooten

readme.txt

@@ -114,6 +114,11 @@ remove_filter( 'boxzilla_box_content', 'do_shortcode' );
 remove_filter( 'boxzilla_box_content', 'shortcode_unautop' );
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+= Where do I report security bugs found in this plugin? =
+
+Please report security bugs found in the source code of the plugin through the [Patchstack Vulnerability Disclosure Program](https://patchstack.com/database/vdp/e5e16c0c-f6c9-469f-a83e-d5342d973d88). The Patchstack team will assist you with verification, CVE assignment, and notify the developers of this plugin.
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 == Screenshots ==
 
 1. A Boxzilla WordPress popup with a newsletter sign-up form.