Re-add mention of XHProf to the docs #377
Description
In #336 the reference to XHProf was replaced with Excimer. I think it's fair to say that Excimer is less well known than XHProf, so I recommend we re-add some mention of XHProf to this page. Something along the lines of:
Flamegraphs are generated with the Excimer PHP extension, using sampling mode. Excimer is a replacement for XHProf with benefit 1 and benefit 2.