docs: create a readthedocs manual

The goal is to remove the WIKI since modifications can not be
submitted via PRs.

docs/howto-develop.rst

@@ -0,0 +1,39 @@
+How to Develop SNM
+==================
+
+Run tests
+---------
+
+You can run the testsuite via
+
+::
+
+   nix-build tests -A extern.nixpkgs_20_03
+   nix-build tests -A intern.nixpkgs_unstable
+   ...
+
+Nixops
+------
+
+You can test the setup via ``nixops``. After installation, do
+
+::
+
+   nixops create nixops/single-server.nix nixops/vbox.nix -d mail
+   nixops deploy -d mail
+   nixops info -d mail
+
+You can then test the server via e.g. \ ``telnet``. To log into it, use
+
+::
+
+   nixops ssh -d mail mailserver
+
+Imap
+----
+
+To test imap manually use
+
+::
+
+   openssl s_client -host mail.example.com -port 143 -starttls imap