Update README with git-send-email example (gopasspw#74)

Signed-off-by: Daniel Gray <[email protected]>

README.md

@@ -65,6 +65,22 @@ git-credential-gopass configure --local --store=ci-team
 
 This puts the value in front of the Gopass search path.
 
+#### Using with SMTP
+
+If you want to use this with [`git-send-email`](https://git-scm.com/docs/git-send-email) you'll need to:
+
+1. [Set Git Credential Helper](#set-git-credential-helper)
+2. Put your password in `.../gopass/stores/root/git/example.com_465/username.gpg`
+3. Include `[sendemail]` like:
+
+   ```text
+   [sendemail]
+	   smtpserver = example.com
+	   smtpuser = username
+	   smtpencryption = ssl
+	   smtpserverport = 465
+   ```
+
 [Gopass]: https://github.com/gopasspw/gopass
 [releases]: https://github.com/gopasspw/git-credential-gopass/releases
 [git credentials]: https://git-scm.com/docs/gitcredentials