Fix the list of configuration constants in README.md

[elig0n]

Fixed the list of available configuration constants that show in the README to match those in the script:

• COMMAND and YANK_COMMAND are removed since they are obsolete with the introduction of the corresponding environment variables.
• Added the constant TMUX_WINDOW_ID
• Did not add HIDE_WINDOW since it appears to be unused: Remove HIDE_WINDOW constant #10