Make signs optional (on by default).

doc/gitgutter.txt

@@ -96,6 +96,7 @@ You can customise:
 - Line highlights
 - Extra arguments for git-diff
 - Whether or not vim-gitgutter is on initially (defaults to on)
+- Whether or not signs are shown (defaults to yes)
 - Whether or not line highlighting is on initially (defaults to off)
 - Whether or not vim-gitgutter runs on `BufEnter` (defaults to yes)
 - Whether or not vim-gitgutter runs for all buffers on `FocusGained` (defaults
@@ -155,6 +156,13 @@ To customise the symbols, add the following to your |vimrc|:
   let g:gitgutter_sign_modified_removed = 'ww'
 <
 
+SIGNS
+
+To never show signs, use this:
+>
+  let git:gitgutter_signs = 0
+<
+
 LINE HIGHLIGHTS
 
 Similarly to the signs' colours, set up the following highlight groups in your