for the sourcerers out there... (#254)

doc/airline.txt

@@ -124,9 +124,13 @@ CUSTOMIZATION                                        *airline-customization*
 The following are some unicode symbols for customizing the left/right
 separators, as well as the powerline font glyths.
 
-Note: You must define the dictionary first before setting values: >
+Note: You must define the dictionary first before setting values. Also, it's a
-  let g:airline_symbols = {}
+good idea to check whether if it exists as to avoid accidentally overwritting
->
+its contents. >
+  if !exists('g:airline_symbols')
+    let g:airline_symbols = {}
+  endif
+
   " unicode symbols
   let g:airline_left_sep = '»'
   let g:airline_left_sep = '▶'