Update README.markdown
Update README.markdown

Updated links for Git and GUI software (if Windows users want to run it), and some typos/grammatical fixes.
sendgrid/docs,sendgrid/docs,Aonepathan/docs,katieporter/docs,Aonepathan/docs,Whatthefoxsays/docs,katieporter/docs,ksigler7/docs,katieporter/docs,Aonepathan/docs,Aonepathan/docs,katieporter/docs,ksigler7/docs,Whatthefoxsays/docs,sendgrid/docs,ksigler7/docs,Aonepathan/docs,katieporter/docs,Whatthefoxsays/docs