Add debugging section to README.

README.mkd

@@ -151,6 +151,14 @@ vim-gitgutter shows removed lines with a sign on the line above.  In this case t
 
 Vim only allows one sign per line.  Before adding a sign to a line, vim-gitgutter checks whether a sign has already been added by somebody else.  If so it doesn't do anything.  In other words vim-gitgutter won't overwrite another plugin's signs.  It also won't remove another plugin's signs.
 
+> Why aren't any signs showing at all?
+
+Here are some things you can check:
+
+* Your git config is compatible with the version of git which your Vim is calling (`:echo system('git --version')`).
+* Your Vim supports signs (`:echo has('signs')` should give `1`).
+* Your file is being tracked by git and has unstaged changes.
+
 
 ### Alternatives