docs: Add READMEs for git, plasma, and tmux

Whether or not Plasma-specific dotfiles are still needed is
questionable, but I will keep this directory for now.

git/README.md

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+# Git
+
+I use git as my main tool for distributed version control.
+
+## Dependencies
+
+- [git][git] - The standard version control system
+
+## Installation
+
+```sh
+make package=git
+```
+
+## Usage
+
+Add a `.gituser` file to your `$HOME` with the following:
+
+```gitconfig
+[user]
+  name = <the name you use for git commits>
+  email = <the email you use for git commits>
+  signingkey = <the subkey you use to sign git commits>
+```
+
+[git]: https://www.archlinux.org/packages/extra/x86_64/git/

plasma/README.md

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+# Plasma
+
+Here lies any dotfiles I use specifically for KDE Plasma (not just KDE).
+
+## Dependencies
+
+- [plasma-desktop][plasma-desktop] - The KDE Plasma desktop environment
+- [python-pywal][python-pywal] - A universal color scheme changer
+
+## Installation
+
+```sh
+make package=plasma
+```
+
+## Usage
+
+Start plasma as you normally would and your previous pywal settings will be automatically applied.
+
+[plasma-desktop]: https://www.archlinux.org/packages/extra/x86_64/plasma-desktop/
+[python-pywal]: https://www.archlinux.org/packages/community/any/python-pywal/

tmux/README.md

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+# tmux
+
+I use tmux to run multiple shells inside a single terminal, in addition to all the other benefits that tmux provides, such as session management, persistent history, and detaching.
+
+## Dependencies
+
+- [tmux][tmux] - Terminal multiplexer
+- [fish][fish] - The friendly interactive shell
+
+## Installation
+
+```sh
+make package=tmux
+```
+
+## Usage
+
+Start tmux as you normally would.
+
+[tmux]: https://www.archlinux.org/packages/community/x86_64/tmux/
+[fish]: https://www.archlinux.org/packages/community/x86_64/fish/