[Feature Request] improve documentation for options

[Jasha10]

The scripts/helpers.sh file defines a lot of configurable options that can be set in one's .tmux.conf file. It would be nice to have exhaustive documentation of these in README.md.

For example, you can do set -g @yank_line 'C-y' in your .tmux.conf to change the yank-line keybinding from prefix + y to prefix + C-y. Currently this is undocumented.

[Jasha10]

Here is some (possibly incomplete?) documentation I've put together:

Mappings defined via tmux-yank:

• y => yank in copy-mode and copy-mode-vi, controlled by @copy_mode_yank
• Y => put in copy-mode and copy-mode-vi, controlled by @copy_mode_put
• M-y => yank&put in copy-mode and copy-mode-vi, controlled by @copy_mode_yank_put
• ! => yank&put w/o newline in copy-mode and copy-mode-vi, controlled by @copy_mode_yank_put_wo_newline
• prefix + y => yank line in normal mode, controlled by @yank_line
• prefix + Y => yank $PWD in normal mode, controlled by @yank_pane_pwd