Add API Documentation

[MikeRyanDev]

Gaps:

• hot and cold - Difference between the two, how to assign values, valid string syntax
• getTestScheduler - Why you might need this when testing streams that require custom schedulers

[cvuorinen]

I came here too to look for documentation & usage examples. I think the empty readme might put off potential users and at least some basic usage examples would be nice. Could also add links to point where users can find more info about marble testing (e.g. https://github.com/ReactiveX/rxjs/blob/master/doc/writing-marble-tests.md).

[mcattle]

I've submitted PR #9 as a starting point for library documentation.

[dolanmiu]

The fact this repo has no formal README or docs made me even question if I've got the right link or not lol

[jacobweber]

+1, especially since the Angular docs are recommending this package. The two links in the readme are either broken or seem to be irrelevant. It would be nice to get at least a basic writeup of what this package does, and how it's different from using https://rxjs-dev.firebaseapp.com/guide/testing/marble-testing.