Custom indicator example

[DaveSkender]

As an adventurous library user, I want to add custom indicators that leverage the library model, so I can use the same conventions with my own indicator.

• add example to Examples.sln
• add guidance to documentation

[DaveSkender]

Also see #622 as another possible opportunity as an example.

[DaveSkender]

Also, consider a skender.stock.indicators.community project, separately, where we can include indicators with lower prominence and quality standards. It would be a more open but lightly curated set of these custom indicators.