update documentation about symbols dictionary (#193).

README.md

@@ -12,6 +12,7 @@ Lean & mean statusline for vim that's light as air.
 *  optimized for speed; it loads in under a millisecond.
 *  extensive suite of themes for popular colorschemes including [solarized][23] (dark and light), [tomorrow][24] (all variants), [base16][32] (all variants), [molokai][25], [jellybeans][26] and others; have a look at the [screenshots][14] in the wiki.
 *  supports 7.2 as the minimum Vim version
+*  predictable release cycle; the master branch is experimental and may break from time to time, if you prefer stability, use the most recent tag which usually is released once every 2 weeks.
 
 # Straightforward customization
 

doc/airline.txt

@@ -122,6 +122,9 @@ 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: >
+  let g:airline_symbols = {}
 >
   " unicode symbols
   let g:airline_left_sep = '»'