Staging selected lines. Issue #57

PR for issue #57 Staging selected lines.

Do you think this covers everything adequately?
Let me know if there's any typos or re-wording you'd prefer or whatever.

I haven't had a chance to add images about staging selected lines. Do you think they would help, or do you think they would cause clutter? As for myself, when I originally found this functionality in Git Extensions I found it quite straightforward to use, so perhaps the instructions are sufficient without additional images. 

I use this feature regularly in my work. I need to find out what I do and raise a gitextensions issue because there's a way to crash Git Extensions - I think by resetting individual lines on a staged file.