PLAT-134 Add Contribution Guidelines to Platform Open Source README files

https://newrelic.atlassian.net/browse/PLAT-1342

PLAT-1342 Add Contribution Guidelines to Platform Open Source README files

https://newrelic.atlassian.net/browse/PLAT-1342

sidekick: @christynicol  
FYI: @serened @lars2893 @tmacie