Added supported browsers readme section
Added supported browsers readme section

Mention of `window.btoa` polyfill for old IE.

craigspaeth/superagent,KuroGuo/suberagent,zsh2938/superagent,thejameskyle/superagent,yussan/superagent,sebastianvera/superagent,ertrzyiks/superagent,thejameskyle/superagent,yeatszhang/superagent,simonvizzini/superagent,ertrzyiks/superagent,visionmedia/superagent,reinaldo13/superagent,focusaurus/superagent,ccapndave/superagent,aaroncrows/superagent,modulexcite/superagent,KingMario/superagent,modulexcite/superagent,focusaurus/superagent,visionmedia/superagent,kmalakoff/superagent-ls,brainbytes/superagent,fredigato/superagent,philipshurpik/superagent,stanzi83/superagent,yussan/superagent,sutoiku/superagent,noikiy/suberagent,fredigato/superagent,stasha/superagent,nemisj/superagent,mvanlonden/superagent-webpack-client,reinaldo13/superagent,soyuka/superagent,visionmedia/superagent,killerlayup/superagent,h1bomb/superagent,drinks/superagent,brotchie/superagent,noikiy/suberagent,RubenVerborgh/superagent,targos/superagent,zsh2938/superagent,alinz/superagent,killerlayup/superagent,drinks/superagent,jarvisaoieong/superagent,andyburke/superagent,stasha/superagent,aaroncrows/superagent,puterakahfi/superagent,nemisj/superagent,puterakahfi/superagent,sutoiku/superagent,ccapndave/superagent,alinz/superagent,craigspaeth/superagent,eligolding/superagent,sebastianvera/superagent,KingMario/superagent,yeatszhang/superagent,wushuyi/superagent,KuroGuo/suberagent,mvanlonden/superagent-webpack-client,geminiyellow/superagent,jarvisaoieong/superagent,kmalakoff/superagent-ls,deltreey/superagent,soyuka/superagent,brotchie/superagent,eligolding/superagent,deltreey/superagent,RubenVerborgh/superagent,stanzi83/superagent,philipshurpik/superagent,wushuyi/superagent,simonvizzini/superagent,h1bomb/superagent,geminiyellow/superagent,targos/superagent,brainbytes/superagent,andyburke/superagent