Adds installation instructions for installing with Nix

[ChristopherHarwell]

What does this change add?

I updated to README to have a one liner shell script for installing tmuxp with Nix package manager.

What does the one-command do?

The one-liner command using Nix to install tmux and tmuxp while ensuring that tmux is installed before installing tmuxp can be explained as follows:

1. The command starts with [[ -z $(which tmux) ]] &&.
   - [[ -z $(which tmux) ]] checks if the command which tmux returns an empty string, indicating that tmux is not found.
   - The double brackets [[ ... ]] are used for conditional tests in shell scripts.
   - The -z flag checks if the string is empty ¹.
   - $(...) is command substitution, which executes the command within the parentheses and replaces it with the output of that command ¹.

2. If tmux is not found, the command executes the following block: echo "tmux not found, please install tmux first".
   - This block displays a message indicating that tmux needs to be installed before proceeding with tmuxp installation.

3. If tmux is found, the command executes the following block: (nix-env -i tmux && nix-env -i tmuxp)².
   - (command1 && command2) executes command1 and then command2 if command1 succeeds.
   - nix-env -i tmux installs tmux using the nix-env command.
   - nix-env -i tmuxp installs tmuxp using the nix-env command.

4. If tmux is already installed, the command executes the following block: nix-env -i tmuxp.
   - This block directly installs tmuxp using the nix-env command.

In summary, the one-liner command checks if tmux is installed, and if not, it displays a message to install tmux first. Otherwise, it installs tmux and then installs tmuxp.

References:

Footnotes

1. github.com - tmux wiki ↩ ↩²

2. github.com - installing tmux + tmuxp with Nix ↩

[CLAassistant]

All committers have signed the CLA.

[tony]

@ChristopherHarwell A very detailed PR description. Well done!

Are you up to signing the CLA before I merge?

[codecov]

Codecov Report

Merging #883 (70a9a31) into master (f794f31) will not change coverage.
Report is 63 commits behind head on master.
The diff coverage is n/a.

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[tony]

@ChristopherHarwell Hi, thanks again for the contribution.

Can you sign the CLA?

[tony]

@ChristopherHarwell Hi! Can you take a look at this?

[ChristopherHarwell]

Yes. I just signed it.

[tony]

@ChristopherHarwell Thank you!

[tony]

@ChristopherHarwell Live in 1.30.1 (PyPI, highlight, current README, highlight)

[tony]

Thank you once again for the contribution!