add --help flag

Running git `cz --help` should include a help message explaining how to use it.

````
$ git-cz --help
/usr/local/lib/node_modules/commitizen/dist/cli/strategies/git-cz.js:87
throw new Error('No files added to staging! Did you forget to run git add?');
^

Error: No files added to staging! Did you forget to run git add?
at /usr/local/lib/node_modules/commitizen/dist/cli/strategies/git-cz.js:87:13
at /usr/local/lib/node_modules/commitizen/dist/commitizen/staging.js:25:5
at ChildProcess.exithandler (child_process.js:263:7)
at emitTwo (events.js:125:13)
at ChildProcess.emit (events.js:213:7)
at maybeClose (internal/child_process.js:927:16)
at Socket.stream.socket.on (internal/child_process.js:348:11)
at emitOne (events.js:115:13)
at Socket.emit (events.js:210:7)
at Pipe._handle.close [as _onclose] (net.js:547:12)
````

首先重現 `git-cz --help`，並透過錯誤中顯示的進入點 `git-cz.js` 追蹤該命令。確認不需要暫存檔案即可顯示說明；該 issue 沒有指出 repository 中的原始碼檔案或要執行的測試，因此必須先找到它們。

由索引模型根據 Issue 內容生成。