a "Coming from Sinon" doc

[searls]

Worth having a doc that just covers differences in td.js vs sinon, especially where the terminology differs. Like it or not, thousands of people learned the word "spy" from sinon, where a spy is not a spy but rather a sort of partial mock / proxy.

Things to cover:

• spies are not spies
• re-emphasis on why test double doesn't offer a partial mock / call-through-to-real-thing mode as sinon spy() does