[add] searchcount's document

CHANGELOG.md

@@ -12,6 +12,7 @@ This is the Changelog for the vim-airline project.
     - [vim-dirvish](https://github.com/justinmk/vim-dirvish) support
     - [fzf.vim](https://github.com/junegunn/fzf.vim) support
     - [OmniSharp](https://github.com/OmniSharp/omnisharp-vim) support
+    - [searchcount](https://vim-jp.org/vimdoc-en/eval.html#searchcount())  support
 - Improvements
   - git branch can also be displayed using [gina.vim](https://github.com/lambdalisue/gina.vim)
   - coc extensions can also show additional status messages

doc/airline.txt

@@ -940,6 +940,12 @@ title accordingly.
 * configure the title text for location list buffers >
   let g:airline#extensions#quickfix#location_text = 'Location'
 <
+-------------------------------------                     *airline-searchcount*
+The searchcount extension supports the searchcount() function in Vim script.
+
+* enable/disable searchcount integration >
+  let g:airline#extensions#searchcount#enabled = 1
+<
 -------------------------------------                    *airline-syntastic*
 syntastic <https://github.com/vim-syntastic/syntastic>