[documentation] More information for provider implementations

[cbowns]

I found it pretty difficult to implement a new provider. Here's a placeholder of some things I want to document for the project:

• Recommended methods implementations for a subclass of ARAnalyticsProvider
• What provider methods ARAnalytics calls, and under what conditions. (i.e. only through breakpoints and digging did I realize how screen views map into the generic event callsite)

[orta]

This could be done by fleshing out: https://github.com/orta/ARAnalytics/blob/master/CONTRIBUTING.md#how-can-i-add-another-analytics-provider