Documentation about the need for `+conceal`

* Make README link to the text file point to the file (not the repo)
* Replace the one 'Jedi-Vim' with 'jedi-vim' in README
* Minor grammar tweak to VIM >= 7.3 requirement in the docs
* Explain the need for `+conceal` in the docs
* Update last-changed date on first line of the docs
* Hint at the need for `+conceal` in the README