tmux online crash course

I've tried and given up multiple times to switch my terminal usage to tmux; none of the websites I had referred to didn't give me a clear idea of what and how. Even fundamental things like the difference between `session` and `window` was never made clear to me.

I found a great online resource that finally got me into tmux -- a set of online videos from `upcase`, although I still had to spend a few hours to understand, struggle in making my own configuration (one of which isn't yet solved). Hurray upcase!

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