docs: Integrate with readthedocs.org

This patch adds configuration to integrate with readthedocs.org, using
mkdocs as rendering engine.

It also does minor documentation updates, to tidy some things up
(clearer titles, move some documentation from the root into docs/, etc).

docs/upgrading.md

@@ -0,0 +1,40 @@
+
+This file contains notes for upgrading between different versions.
+
+
+## 0.07 → 1.0
+
+No backwards-incompatible changes. No more are expected within this major
+version.
+
+
+## 0.06 → 0.07
+
+No backwards-incompatible changes.
+
+
+## 0.05 → 0.06
+
+No backwards-incompatible changes.
+
+
+## 0.04 → 0.05
+
+No backwards-incompatible changes.
+
+
+## 0.03 → 0.04
+
+No backwards-incompatible changes.
+
+
+## 0.02 → 0.03
+
+* The default MTA binary has changed. It's now maildrop by default.
+  If you relied on procmail being the default, add the following to
+  /etc/chasquid/chasquid.conf: `mail_delivery_agent_bin: "procmail"`.
+
+* chasquid now listens on a third port, submission-on-TLS.
+  If using systemd, copy the `etc/systemd/system/chasquid-submission_tls.socket`
+  file to `/etc/systemd/system/`, and start it.
+