Single change to `tmux/tmux.conf` file at the end to source

[flakrat]

$HOME/.tmux.conf.local to allow for local modifications to be made without having to modify the base code.

The install.sh file will create an empty $HOME/.tmux.conf.local file if it doesn't already exist.

Updated the readme to include a note about putting local changes into the appropriate file

On branch initial-changes

• Changes to be committed:
  • modified: install.sh
  • modified: readme.md
  • modified: tmux/tmux.conf
  • new file: tmux/tmux.conf.local