docs: Add a "Clients" document

We've had a couple of reported issues about the difficulty of setting up
new clients, or confusion due to using broken clients:

- #46
- #52

This patch adds the first version of a "Clients" document that includes
requirements for all clients, configuration examples, and a list of
known-problematic client software.

The goal is to help reduce friction and confusion when setting up
clients.

The document needs more polishing and examples, which hopefully will be
added later.

Fixes #48.

.mkdocs.yml

@@ -24,6 +24,7 @@ nav:
 - All:
     - aliases.md
     - hooks.md
+    - clients.md
     - dovecot.md
     - dkim.md
     - haproxy.md

docs/clients.md

@@ -0,0 +1,54 @@
+
+# Clients
+
+chasquid supports most SMTP clients, but requires them to have some features:
+
+- Support TLS (either
+  [STARTTLS](https://datatracker.ietf.org/doc/html/rfc3207) or
+  [implicit TLS](https://datatracker.ietf.org/doc/html/rfc8314#section-3.3))
+- Support the
+  [PLAIN authentication method](https://datatracker.ietf.org/doc/html/rfc4954#section-4).
+
+All modern clients should support both, and thus have no problems talking to
+chasquid.
+
+
+## Configuration examples
+
+### [msmtp](https://marlam.de/msmtp/)
+
+This example is useful as either per-user `~/.msmtprc` or system-wide
+`/etc/msmtprc`:
+
+```
+account default
+tls on
+auth on
+
+# Use the SMTP submission port. Many providers block communications to the
+# default port 25, but the submission port 587 tends to work just fine.
+port 587
+
+# Server hostname.
+host SERVER
+
+# Your username (including the domain).
+user USER@DOMAIN
+
+# Your password.
+password SECRET
+```
+
+Replace the `SERVER`, `USER@DOMAIN` and `SECRET` strings with the appropriate
+values.
+
+
+## Problematic clients
+
+These clients are known to have issues talking to chasquid:
+
+- [ssmtp](https://packages.debian.org/source/unstable/ssmtp): does not
+  support the PLAIN authentication method. It is also unmaintained.  
+  Please use [msmtp](https://marlam.de/msmtp/) instead.
+
+