Update README to include new Vim plugin (to browse docs without leaving Vim)

[girishji]

Include the following Vim plugin in README: https://github.com/girishji/devdocs.vim

With this plugin you can:

• browse documents within Vim
• navigate links (like in a real browser)
• install/uninstall slugs
• use VIm as a pager (TUI)

None of the other Vim/Neovim plugins have all these features.

[girishji]

Please merge this PR. It has been 2 months.