Update and document .gitignore
Update and document .gitignore

andriykonoz/sagan,dlizarra/sagan,yhj630520/sagan,mbateman/sagan,martinlippert/sagan,gregturn/sagan,andriykonoz/sagan,mbateman/sagan,seoilhyun/sagan,vizewang/sagan,dharmaraju/sagan,lowtalker/sagan,andriykonoz/sagan,lowtalker/sagan,dvaughn555/sagan,TabberGit/sagan,spring-io/sagan,garylgh/sagan,spring-io/sagan,dkushner/sagan,martinlippert/sagan,yhj630520/sagan,dlizarra/sagan,rwinch/sagan,dharmaraju/sagan,seoilhyun/sagan,garylgh/sagan,TabberGit/sagan,vizewang/sagan,rwinch/sagan,amit-siddhu/sagan,dlizarra/sagan,yhj630520/sagan,eonezhang/sagan,vizewang/sagan,amit-siddhu/sagan,spring-io/sagan,garylgh/sagan,dsyer/sagan,BHASKARSDEN277GITHUB/sagan,martinlippert/sagan,dvaughn555/sagan,dvaughn555/sagan,TabberGit/sagan,andriykonoz/sagan,dharmaraju/sagan,vizewang/sagan,spring-io/sagan,gregturn/sagan,eonezhang/sagan,dlizarra/sagan,martinlippert/sagan,dvaughn555/sagan,dsyer/sagan,eonezhang/sagan,dharmaraju/sagan,dkushner/sagan,yhj630520/sagan,garylgh/sagan,mbateman/sagan,rwinch/sagan,dkushner/sagan,lowtalker/sagan,BHASKARSDEN277GITHUB/sagan,mbateman/sagan,amit-siddhu/sagan,seoilhyun/sagan,gregturn/sagan,rwinch/sagan,BHASKARSDEN277GITHUB/sagan,dkushner/sagan,dsyer/sagan,BHASKARSDEN277GITHUB/sagan,eonezhang/sagan,seoilhyun/sagan,dsyer/sagan,lowtalker/sagan,TabberGit/sagan,amit-siddhu/sagan