Document cmd.exe vulnerability where it can execute the wrong git.

See #508, #538.

doc/gitgutter.txt

@@ -79,6 +79,19 @@ Copy vim-gitgutter's subdirectories into your vim configuration directory:
 See |add-global-plugin|.
 
 
+WINDOWS                                                     *gitgutter-windows*
+
+I recommend configuring vim-gitgutter with the full path to your git executable.
+For example:
+>
+    let g:gitgutter_git_executable = 'C:\Program Files\Git\bin\git.exe'
+<
+This is to avoid a problem which occurs if you have file named "git.*" (i.e.
+with any extension in "PATHEXT") in your current folder.  "cmd.exe" prioritises
+the current folder over folders in 'PATH' and will try to execute your file
+instead of the "git" binary.
+
+
 ===============================================================================
 COMMANDS                                                   *gitgutter-commands*