added text in asynch section for the library overview
added text in asynch section for the library overview

shane-kerr/getdns,smogali/getdns,dkg/getdns,smogali/getdns,glenwiley/getdns,shane-kerr/getdns,glenwiley/getdns,huitema/getdns,smogali/getdns,dkg/getdns,shane-kerr/getdns,dkg/getdns,glenwiley/getdns,Acidburn0zzz/getdns,Acidburn0zzz/getdns,shane-kerr/getdns,huitema/getdns,smogali/getdns,Acidburn0zzz/getdns,dkg/getdns,glenwiley/getdns,Acidburn0zzz/getdns,huitema/getdns,huitema/getdns