Add a  note about UA string, and link to request headers.
Add a  note about UA string, and link to request headers.

paulirish/devopera,SelenIT/devopera,michaelstewart/devopera,kenarai/devopera,operasoftware/devopera,SelenIT/devopera,erikmaarten/devopera,andreasbovens/devopera,paulirish/devopera,paulirish/devopera,payeldillip/devopera,operasoftware/devopera,SelenIT/devopera,kenarai/devopera,simevidas/devopera,shwetank/devopera,cvan/devopera,Jasenpan1987/devopera,Jasenpan1987/devopera,initaldk/devopera,simevidas/devopera,kenarai/devopera,andreasbovens/devopera,operasoftware/devopera,initaldk/devopera,initaldk/devopera,michaelstewart/devopera,simevidas/devopera,michaelstewart/devopera,erikmaarten/devopera,shwetank/devopera,cvan/devopera,cvan/devopera,simevidas/devopera,initaldk/devopera,payeldillip/devopera,michaelstewart/devopera,Jasenpan1987/devopera,shwetank/devopera,erikmaarten/devopera,payeldillip/devopera,payeldillip/devopera,Jasenpan1987/devopera,cvan/devopera,erikmaarten/devopera,SelenIT/devopera,paulirish/devopera,operasoftware/devopera,andreasbovens/devopera,kenarai/devopera