Add completions and documents

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

autoload/requirements.vim

@@ -0,0 +1,44 @@
+""
+" @section Introduction, intro
+" @library
+" <doc/@plugin(name).txt> is generated by <https://github.com/google/vimdoc>.
+" See <README.md> for more information about installation and screenshots.
+
+""
+" Update cache.
+function! requirements#update_cache() abort
+    let s:items = []
+    let l:pip_items = split(system('COMP_WORDS="pip install -" COMP_CWORD=2 PIP_AUTO_COMPLETE=1 pip'))
+    let l:pypi_items = split(system('pip-cache pkgnames'))
+    for l:item in l:pip_items
+        let s:items += [{'word': l:item, 'menu': 'pip'}]
+    endfor
+    for l:item in l:pypi_items
+        let s:items += [{'word': l:item, 'menu': 'pypi'}]
+    endfor
+    call writefile([json_encode(s:items)], s:cache)
+endfunction
+
+if exists('*stdpath')
+    let s:cache_dir_home = stdpath('cache')
+else
+    let s:cache_dir_home = $HOME . '/.cache/nvim'
+endif
+let s:cache_dir = s:cache_dir_home . '/requirements.vim'
+call mkdir(s:cache_dir, 'p')
+""
+" Completion cache path.
+call g:requirements#utils#plugin.Flag('g:requirements#cache',
+            \ s:cache_dir . '/requirements.json'
+            \ )
+let s:cache = g:requirements#cache
+try
+    let s:items = json_decode(readfile(s:cache)[0])
+catch /\v^Vim%(\(\a+\))?:E(684|484|491):/
+    call requirements#update_cache()
+    let s:items = json_decode(readfile(s:cache)[0])
+endtry
+""
+" Completion cache contents. For program.
+call g:requirements#utils#plugin.Flag('g:requirements#items', s:items)
+" vim: et sw=4 ts=4 sts=4: