[Suggestion] Reference documentation txt in Readme.md

[corneliusroemer]

I was struggling to find surround.vim's documentation. It seemed to me like there was nothing but the Readme.md with a few examples.

In fact, there is good documentation, but it's hidden away inside doc/surround.txt which isn't, to my big surprise, referenced in the Readme.

Please add a documentation section to Readme.md referencing surround.txt. Thank you! I love the plugin (on VS Code).

Here's a direct link if anyone comes across this from Google: https://github.com/tpope/vim-surround/blob/master/doc/surround.txt