Add a vim help file

[jonasdiemer]

so I can do :help sexy_scroller to look up all the settings etc.

[joeytwiddle]

I have moved the docs into the appropriate file, but I must have done something wrong because Vim cannot find it when I do:

:help sexy_scroller

If you have any idea why, please let me know!

Originally I wanted the doc:

• In the script, for users who download from www.vim.org/script. I have since relaxed this requirement as I don't think there are many users who do that these days.
• In README.txt for a fast introduction on the Github repo page.
• In the doc/ folder, so it can be accessed from Vim's inline :help. (This is not working for me!)

(As an aside, I note the vimdoc project. That would allow us to write comments in the code, which could then be automatically exported to the other two files.)

[joeytwiddle]

OK got it. I needed to generate the tags file by doing this in the root of the repository:

vim -c "helptags doc/ | quit"