[add] short_path's doc in airline

CHANGELOG.md

@@ -22,6 +22,7 @@ This is the Changelog for the vim-airline project.
 - Other
   - Introduce Vim script static analysis using [reviewdog](https://github.com/reviewdog/action-vint)
   - Added multiple Vim versions to unit tests using Travis CI
+  - Added option to show short paths in the status line
 
 ## [0.11] - 2019-11-10
 - New features

doc/airline.txt

@@ -287,6 +287,10 @@ values):
   To force updating the tabline on mode changes, call `airline#check_mode()`
   in your custom statusline setting: `:set stl=%!airline#check_mode(winnr())`
   will correctly update the tabline on mode changes.
+
+* Display a short path in statusline: >
+  let g:airline_stl_path_style = 'short'
+>
 =============================================================================
 COMMANDS                                                  *airline-commands*