Improve documentation how to write addins

[pascalberger]

We should improve documentation how to create new addins for Cake. Currently we have some documentation here: https://cakebuild.net/docs/extending/addins/. We should add more detailed documentation how to write addins.

This includes mentioning of https://github.com/WormieCorp/generator-cake-addin and https://github.com/WormieCorp/Cake.Addin.Analyzer.

Also should mention Cake Contrib and guidelines etablished there: https://github.com/cake-contrib/Home/blob/master/Guidelines.md, https://github.com/cake-contrib/CakeContrib.Guidelines

TODO:

• [x] Add best practices
• [ ] Add instructions how to create addin from scratch, including generator options
• [ ] Add documentation about available analyzers

[pascalberger]

See also https://github.com/cake-build/website/issues/368

[pascalberger]

See also https://github.com/cake-build/website/issues/151