DOC add some figures

	new file:   cannot_send_pr.png
	new file:   custom_data_cannot_send_pr.png
	new file:   figure_source.pptx
	new file:   fork.png
	new file:   gh-pages.png
	new file:   local.png
	new file:   new_branch_for_pr.png
	new file:   new_commits.png
	new file:   origin_upstream.png
	new file:   send_pr.png
	new file:   step3.png