Add get.sh script for semi-automated installation

Given amount of steps actually needed to install maddy, it makes sense
to automate at least some of them.

Also, since maddy repo is replicated on foxcpp.dev/maddy,
it can be used like this:
curl https://foxcpp.dev/maddy/get.sh | bash

That's, of course, is a partial solution. In future, package
repository least for Debian will be created with properly built
packages.

Response to possible complaints about curl|bash:
There is not that much difference between that and cloning repo
to run 'make install'. People who care can inspect the script
either way and ones who don't care... well, don't care.

.gitignore

@@ -34,3 +34,5 @@ cmd/maddy-*-helper/maddy-*-helper
 # in repo directory.
 cmd/maddy/*mtasts-cache
 cmd/maddy/*queue
+
+maddy-setup/