Add support for flog with git status summary

doc/airline.txt

@@ -231,6 +231,7 @@ values):
       \ 'coc-explorer':  [ 'CoC Explorer', '' ],
       \ 'defx':  ['defx', '%{b:defx.paths[0]}'],
       \ 'fugitive': ['fugitive', '%{airline#util#wrap(airline#extensions#branch#get_head(),80)}'],
+      \ 'floggraph':  [ 'Flog', '%{get(b:, "flog_status_summary", "")}' ],
       \ 'gundo': [ 'Gundo', '' ],
       \ 'help':  [ 'Help', '%f' ],
       \ 'minibufexpl': [ 'MiniBufExplorer', '' ],
@@ -606,6 +607,13 @@ characters.
 
   let g:airline#extensions#branch#vcs_checks = ['untracked', 'dirty']
 <
+-------------------------------------                      *airline-flog*
+vim-flog <https://github.com/rbong/vim-flog>
+
+If vim-flog is installed, vim-airline will display the branch name
+together with a status summary in the git log graph buffer;
+either 'no changes' or the number of added/removed/modified files.
+
 -------------------------------------                   *airline-bufferline*
 vim-bufferline <https://github.com/bling/vim-bufferline>