explain how to cache menu blocks with ESI

[lsmith77]

especially due to the performance issues with MenuBundle it would be good to explain how to efficiently cache menu inside an ESI block.

for now there are two strategies to still be able to handle highlighting the selected menu entry:

1. pass the url to the ESI prevent KnpMenu from loading the entire menu/route/content tree menu-bundle#19 (comment)
2. do the highlighting with javascript

[lsmith77]

we have reduced the performance issues but would still be good to explain this.