Add completions and documents

Use `vimdoc .` to generate documentation.
User can use plugin manager to do it automatically when update plugin.

autoload/coc/source/requirements.vim

@@ -0,0 +1,18 @@
+""
+" https://github.com/neoclide/coc.nvim/wiki/Create-custom-source
+function! coc#source#requirements#init() abort
+    return {
+                \ 'shortcut': 'pip',
+                \ 'priority': 9,
+                \ 'filetypes': ['requirements'],
+                \ }
+endfunction
+
+""
+" https://github.com/neoclide/coc.nvim/wiki/Create-custom-source
+"
+" Completion pip option and PYPI package names.
+function! coc#source#requirements#complete(opt, cb) abort
+    call a:cb(g:requirements#items)
+endfunction
+" vim: et sw=4 ts=4 sts=4: