add basic sendmail email command

chat/email.md

@@ -0,0 +1,19 @@
+# Sendmail
+
+Compose a message like this:
+
+```
+To: alice@domain.com
+Subject: my new mail server
+From: literally@anything.com
+
+This is the message
+
+```
+
+... and put it in ~/mail.txt.
+Then send with sendmail using:
+
+> sendmail ~/mail.txt
+
+