Added: getting started and keybindings in README.md

It would have saved 15min of my time, so I think that it is worth mentioning.

Related:
https://github.com/wuub/SublimeREPL/issues/120

In short, it's adding:

#### Getting started

* Create or open your file with code.
* Menu / Tools / Command Palette (OS X: `⇧⌘P`) 
then type "SublimeREPL" and select the approperiate language.
* Menu / View / Layout / Rows: 2 (OS X: `⌥⇧⌘2`).
* Menu / View / Move File to Group / Group 2 (`⌃⇧2`).

#### Keybindings

* Evaluate in REPL:
 * `⌃+,, s` Selection
 * `⌃+,, f` File  
 * `⌃+,, l` Lines
 * `⌃+,, b` Block
* Transfer in REPL (just copy, without evaluating it):
 * `⌃⇧+,, s` Selection
 * `⌃⇧+,, f` File  
 * `⌃⇧+,, l` Lines
 * `⌃⇧+,, b` Block

Note: `⌃+,, f` means: press Ctrl and Comma, release all, press F.