Add es5 polyfill requirement to the README
Add es5 polyfill requirement to the README

This adds a sub-section to the Installation section that lets users
know that they need to add ember-cli-es5-shim to their project
if they are using PhantomJS < 2.0 because of the use of 
`.bind` in Mirage.

The credit for this really goes to @samselikoff for helping me
with this! 😁
ronco/ember-cli-mirage,cibernox/ember-cli-mirage,cs3b/ember-cli-mirage,LevelbossMike/ember-cli-mirage,jamesdixon/ember-cli-mirage,blimmer/ember-cli-mirage,mydea/ember-cli-mirage,martinmaillard/ember-cli-mirage,ghedamat/ember-cli-mirage,HeroicEric/ember-cli-mirage,constantm/ember-cli-mirage,escobera/ember-cli-mirage,martinmaillard/ember-cli-mirage,seawatts/ember-cli-mirage,unchartedcode/ember-cli-mirage,alecho/ember-cli-mirage,jamesdixon/ember-cli-mirage,makepanic/ember-cli-mirage,jherdman/ember-cli-mirage,flexyford/ember-cli-mirage,jerel/ember-cli-mirage,lazybensch/ember-cli-mirage,ballPointPenguin/ember-cli-mirage,escobera/ember-cli-mirage,seawatts/ember-cli-mirage,samselikoff/ember-cli-mirage,alvinvogelzang/ember-cli-mirage,alvinvogelzang/ember-cli-mirage,samselikoff/ember-cli-mirage,jherdman/ember-cli-mirage,oliverbarnes/ember-cli-mirage,HeroicEric/ember-cli-mirage,alecho/ember-cli-mirage,samselikoff/ember-cli-mirage,lependu/ember-cli-mirage,mixonic/ember-cli-mirage,mixonic/ember-cli-mirage,kagemusha/ember-cli-mirage,unchartedcode/ember-cli-mirage,ghedamat/ember-cli-mirage,ballPointPenguin/ember-cli-mirage,LevelbossMike/ember-cli-mirage,jherdman/ember-cli-mirage,makepanic/ember-cli-mirage,oliverbarnes/ember-cli-mirage,IAmJulianAcosta/ember-cli-mirage,IAmJulianAcosta/ember-cli-mirage,lazybensch/ember-cli-mirage,ibroadfo/ember-cli-mirage,ronco/ember-cli-mirage,mydea/ember-cli-mirage,jherdman/ember-cli-mirage,unchartedcode/ember-cli-mirage,kategengler/ember-cli-mirage,constantm/ember-cli-mirage,lependu/ember-cli-mirage,blimmer/ember-cli-mirage,kagemusha/ember-cli-mirage,kategengler/ember-cli-mirage,samselikoff/ember-cli-mirage,jerel/ember-cli-mirage,cibernox/ember-cli-mirage,ibroadfo/ember-cli-mirage,unchartedcode/ember-cli-mirage,cs3b/ember-cli-mirage,flexyford/ember-cli-mirage