Add .well-known/security.txt file

Merge branch 'pr-247'
Closes QubesOS/qubes-issues#7022

Author: unman

.well-known/security.txt

@@ -0,0 +1,6 @@
+Contact: https://www.qubes-os.org/security/
+Expires: 2028-01-01T08:00:00.000Z
+Encryption: https://keys.qubes-os.org/keys/qubes-os-security-team-key.asc
+Canonical: https://qubes-os.org/.well-known/security.txt
+Policy: https://www.qubes-os.org/security/
+

_config.yml

@@ -44,3 +44,5 @@ allowed_categories:
 
 default_author: "The Qubes team"
 default_category: "announcements"
+
+include: ['.well-known']