DOC add lost swc-shell images from swcarpentry/guide repo

	new file:   lessons/swc-shell/absolute_path.png
	new file:   lessons/swc-shell/command_shell.svg
	new file:   lessons/swc-shell/decwriter.jpg
	new file:   lessons/swc-shell/direct_shell_usage.png
	new file:   lessons/swc-shell/filedir_challenge.png
	new file:   lessons/swc-shell/find_file_tree.png
	new file:   lessons/swc-shell/google_vs_grep.png
	new file:   lessons/swc-shell/nano.png
	new file:   lessons/swc-shell/nano_quotation.png
	new file:   lessons/swc-shell/permissions_table.png
	new file:   lessons/swc-shell/process_stdin_stdout.png
	new file:   lessons/swc-shell/public_private_keys.png
	new file:   lessons/swc-shell/relative_path.png
	new file:   lessons/swc-shell/remote_shell_usage.png
	new file:   lessons/swc-shell/running_a_process.png
	new file:   lessons/swc-shell/running_wc.png
	new file:   lessons/swc-shell/running_wc_sort.png
	new file:   lessons/swc-shell/running_wc_sort_head.png
	new file:   lessons/swc-shell/shell_as_process.png
	new file:   lessons/swc-shell/shell_on_shell.png
	new file:   lessons/swc-shell/x_for_directories.png