Add git stash tips

Guides/git.md

@@ -32,6 +32,12 @@ You can replace `--global` by `--local` to configure a specific project.
 
 `git rm --cached <file>` remove a file from git but keep this file on drive
 
+### Stashing
+
+`git stash -u -m 'Experiment optimized code`
+
+`-u` is used to stash _untracked files_
+
 ### Diff tool
 
 `git difftool` use a tool like _vimdiff_