GPG command

As authored by @fordhurley.
We had a little merge problem, so had to recreate this file on master.

pages/common/gpg.md

@@ -0,0 +1,19 @@
+# gpg
+
+> Gnu Privacy Guard
+
+- sign doc.txt without encryption (writes output to doc.txt.asc)
+
+`gpg --clearsign {{doc.txt}}`
+
+- encrypt doc.txt for alice@example.com (output to doc.txt.gpg)
+
+`gpg --encrypt --recipient {{alice@example.com}} {{doc.txt}}`
+
+- encrypt doc.txt with only a passphrase (output to doc.txt.gpg)
+
+`gpg --symmetric {{doc.txt}}`
+
+- decrypt doc.txt.gpg (output to STDOUT)
+
+`gpg --decrypt {{doc.txt.gpg}}`