README: wrap at 79, improved formatting
README: wrap at 79, improved formatting

Update the README doc to wrap at 79 characters where possible and
improve formatting to better match the way it will actually be rendered
with Markdown. For example:

One statement.
Another statement.

Is rendered with Markdown as:

One statement. Another statement.

Tested with grip markdown renderer.

natemurthy/marathon,airbnb/marathon,sttts/marathon,drewrobb/marathon,timcharper/marathon,gsantovena/marathon,rtward/marathon,c089/marathon,gsantovena/marathon,titosand/marathon,cherrydocker/marathon-1,okuryu/marathon,EvanKrall/marathon,bobrik/marathon,EvanKrall/marathon,hangyan/marathon,spacejam/marathon,bobrik/marathon,Kosta-Github/marathon,felixb/marathon,sttts/marathon,sielaq/marathon,IanSaunders/marathon,14Zen/marathon,felixb/marathon,titosand/marathon,quamilek/marathon,janisz/marathon,ayouwei/marathon,sielaq/marathon,MrMarvin/marathon,HardikDR/marathon,drewrobb/marathon,sledigabel/marathon,murat-lacework/marathon,matsluni/marathon,rtward/marathon,vivekjuneja/marathon,ss75710541/marathon,meln1k/marathon,meln1k/marathon,Caerostris/marathon,okuryu/marathon,pgkelley4/marathon,quamilek/marathon,bsideup/marathon,gsantovena/marathon,sttts/marathon,meln1k/marathon,guenter/marathon,MrMarvin/marathon,drewrobb/marathon,bobrik/marathon,spacejam/marathon,timcharper/marathon,EasonYi/marathon,hangyan/marathon,rtward/marathon,14Zen/marathon,gsantovena/marathon,airbnb/marathon,sepiroth887/marathon,14Zen/marathon,Caerostris/marathon,EasonYi/marathon,ayouwei/marathon,bsideup/marathon,sepiroth887/marathon,guenter/marathon,cgvarela/marathon,hangyan/marathon,airbnb/marathon,pugna0/marathon,ayouwei/marathon,quamilek/marathon,mikejihbe/marathon,timcharper/marathon,MiLk/marathon,yp-engineering/marathon,mesosphere/marathon,Caerostris/marathon,MrMarvin/marathon,janisz/marathon,meln1k/marathon,EvanKrall/marathon,c089/marathon,felixb/marathon,mikejihbe/marathon,guenter/marathon,c089/marathon,natemurthy/marathon,mesosphere/marathon,natemurthy/marathon,manojlds/marathon,sepiroth887/marathon,murat-lacework/marathon,ramitsurana/marathon,meln1k/marathon,SivagnanamCiena/marathon,HardikDR/marathon,lelezi/marathon,mesosphere/marathon,janisz/marathon,SivagnanamCiena/marathon,manojlds/marathon,janisz/marathon,murat-lacework/marathon,pugna0/marathon,spacejam/marathon,ramitsurana/marathon,guenter/marathon,Yhgenomics/marathon,mikejihbe/marathon,yp-engineering/marathon,manojlds/marathon,gsantovena/marathon,felixb/marathon,guenter/marathon,cherrydocker/marathon-1,sttts/marathon,Yhgenomics/marathon,Caerostris/marathon,lelezi/marathon,airbnb/marathon,HardikDR/marathon,c089/marathon,Kosta-Github/marathon,EasonYi/marathon,MiLk/marathon,IanSaunders/marathon,pugna0/marathon,sielaq/marathon,ramitsurana/marathon,sledigabel/marathon,sledigabel/marathon,mesosphere/marathon,mesosphere/marathon,cherrydocker/marathon-1,MiLk/marathon,ss75710541/marathon,vivekjuneja/marathon,spacejam/marathon,ss75710541/marathon,Yhgenomics/marathon,janisz/marathon,bobrik/marathon,cgvarela/marathon,okuryu/marathon,SivagnanamCiena/marathon,vivekjuneja/marathon,drewrobb/marathon,sielaq/marathon,matsluni/marathon,14Zen/marathon,bsideup/marathon,EvanKrall/marathon,lelezi/marathon,matsluni/marathon,pgkelley4/marathon,Caerostris/marathon,IanSaunders/marathon,cgvarela/marathon