Add "howto write a plugin" documentation

[tiran]

Custodia is easily extensible with external plugins but it's not yet well documented. I created an example project that can be used to bootstrap a new plugin, https://github.com/tiran/custodia-example-plugins

We could consider to add my example plugings to latchset or even turn it into a Cookie Cutter (https://github.com/audreyr/cookiecutter) recipe.