docs: Fix formatting of security document
Markdown considers a block of indented text after a list item to be part
of that list item.  Insert a small piece of text at the top level to
prevent that.
@@ -11,6 +11,8 @@ project:
- David Brown: davidb@davidb.org or david.brown@linaro.org
- Fabio Utzig: utzig@apache.org
+If you wish to send encrypted email, you may use these PGP keys:
+
pub rsa4096 2011-10-14 [SC]
DAFD760825AE2636AEA9CB19E6BA9F5C5E54DF82
uid [ultimate] David Brown <davidb@davidb.org>
