Add docs for TableEditingController

[jessesquires]

Follow up from #67.

We need to add docs to the new TableEditingController.

https://github.com/jessesquires/JSQDataSourcesKit/blob/develop/Source/TableEditingController.swift

[jessesquires]

cc @psartzetakis 😄

[psartzetakis]

I will take care of it 😄

[psartzetakis]

@jessesquires , apologies but I haven't done that before. Would you mind giving some pointers / tips ? Do I have just to add comments in the source file?

[jessesquires]

Hey @psartzetakis - no problem! 😄

Yeah, that's all it is — just comments. Just follow the existing examples and feel free to comment here if you have any questions. (Or we can talk on the PR, too)