Add comments for color customization instructions on readme

Author: tiagofumo

README.md

@@ -57,13 +57,13 @@ let s:rspec_red = 'FE405F'
 let s:git_orange = 'F54D27'
 
 let g:NERDTreeExtensionHighlightColor = {} " this line is needed to avoid error
-let g:NERDTreeExtensionHighlightColor['css'] = s:blue " sets the color of .css files to blue
+let g:NERDTreeExtensionHighlightColor['css'] = s:blue " sets the color of css files to blue
 
 let g:NERDTreeExactMatchHighlightColor = {} " this line is needed to avoid error
-let g:NERDTreeExactMatchHighlightColor['.gitignore'] = s:git_orange
+let g:NERDTreeExactMatchHighlightColor['.gitignore'] = s:git_orange " sets the color for .gitignore files
 
 let g:NERDTreePatternMatchHighlightColor = {} " this line is needed to avoid error
-let g:NERDTreePatternMatchHighlightColor['.*_spec.rb'] = s:rspec_red
+let g:NERDTreePatternMatchHighlightColor['.*_spec.rb'] = s:rspec_red " sets the color files ending with _spec.rb
 
 ```