Documentation: Create an Extending H2O section in the docs

Description

Let's create a new section of the docs for developers who want to customize, extend or contribute to H2O. Maybe we call it "Extending H2O"? and can help with the content.

There was a discussion on Gitter about how to add a new algo and get it to appear in Flow:

Assignee

Navdeep

Reporter

Erin LeDell