Sync a GitHub fork with the upstream manually

This is specific to aws-sdk-react-native, since that is the primary fork I sync.

                # add upstream if it doesn't exist 
                # https://help.github.com/articles/configuring-a-remote-for-a-fork/#platform-mac  
                git remote add upstream git@github.com:awslabs/aws-sdk-react-native.git  
                git remote -v  
                
                # sync upstream # https://help.github.com/articles/syncing-a-fork/  
                git fetch upstream  
                git checkout master  
                git merge upstream/master  
                git push origin master