Added security.txt.

SECURITY.txt

@@ -0,0 +1,30 @@
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA256
+
+#
+# Please report security issues as below, specifically
+# mentioning NGINX Unit in the subject.
+#
+Canonical: https://unit.nginx.org/.well-known/security.txt
+Contact: mailto:security-alert@nginx.org
+Encryption: https://nginx.org/keys/maxim.key
+Encryption: https://nginx.org/keys/sb.key
+Encryption: https://nginx.org/keys/thresh.key
+Expires: 2024-01-01T00:00:00.000Z
+Policy: https://www.first.org/cvss/v3.1/specification-document
+Preferred-Languages: en
+
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