I find myself having to write a custom plugin, and it's really hard to know what I should be doing in each method of BasePlugin. It would be nice if there was comprehensive API documentation aimed at plugin authors, published online. Ideally it would cover: * Which methods of BasePlugin can be overridded. * What plugin authors should do in each method. * which attributes are set on the BasePlugin, and what they're for (installdir, builddir etc). * When each method on BasePlugin will get called. * Functions, classes and methods from the rest of snapcraft that's considered a public, stable API.