Add “Combining reducers” recipe

[gaearon]

Anybody wants to write this?

It should explain:

1. You don't have to use combineReducers(), and you can write top-level reducer by hand
2. If you like it, you can use combineReducers() multiple times, like in shopping-cart example
3. The result of combineReducer() doesn't have to be top level reducer. You can tweak how it works, e.g. Add store.resetState for reverting to initial state value #658 (comment)

[IwanKaramazow]

I'll take a crack at this over the weekend, if no one is currently writing this.

[IwanKaramazow]

Started working on this over weekend, will probably take me at least two weeks due to my heavy academic workload. I'll report back when I have something of more substance.

[sai-prasanna]

@gaearon I can write this. Which file should I update in the docs with the contents?

[markerikson]

@IwanKaramazow , @sai-prasanna : I'm looking at trying to put together a more comprehensive doc on organizing reducer logic. See #1784. Would appreciate any thoughts, comments, or help :)

[timdorr]

Closing out in favor of the efforts Mark is championing over at #1930.