set_mark documentation

[jackmac92]

I'm not sure what set_mark etc are supposed to do, I can make a PR for the README out of your answer here if that would help!

[Lenbok]

with_mark and with_or_set_mark act with respect to a state variable know as the mark (which can be set to true or false via set_mark).

When the mark is set, these functions output the specified keys with the SHIFT modifier also pressed. When the mark is not set, they output the specified keys without additional modifier. These let you emulate Emacs' transient mark mode, where after setting the mark and moving around, you are selecting text between the mark position and the current cursor position.