docs: Add READMEs for code, gpg, and systemd

Note that systemd usage information needs to be revised since systemd
user units appear to be enabled by default, and may be started by
default when placed in the default.target.wants directory.

code/README.md

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+# Code
+
+Code is a GUI text editor.
+
+## Dependencies
+
+- [code][code] - The open source build of a popular non-free text editor
+
+## Installation
+
+```sh
+make package=code
+```
+
+## Usage
+
+Install the extensions I use with:
+
+```sh
+make code-extensions
+```
+
+Then, start Code as you normally would.
+
+[code]: https://www.archlinux.org/packages/community/x86_64/code/

gpg/README.md

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+# GPG
+
+GPG is used to sign and encrypt things.
+
+## Dependencies
+
+- [`gnupg`][gnupg] - The GNU Privacy Guard
+
+## Installation
+
+```sh
+make package=gpg
+```
+
+## Usage
+
+Use gpg as you normally would. Make sure that you export `GPG_TTY` in your shell's init script. This is how you would do it in [fish](/fish):
+
+```fish
+export GPG_TTY=(tty)
+```
+
+[gnupg]: https://www.archlinux.org/packages/core/x86_64/gnupg/

systemd/README.md

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+# systemd
+
+*systemd* is described as "a suite of basic building blocks for a Linux system". One of the functions it can be used for is starting daemons at login.
+
+## Dependencies
+
+- [openssh][openssh] - `ssh-agent.service` support
+- [rxvt-unicode-patched][rxvt-unicode-patched] - `urxvtd.service` support
+- [mpd][mpd] - `mpd.service` support
+
+## Installation
+
+```sh
+make package=systemd
+```
+
+## Usage
+
+To start all the user services I use, run:
+
+```sh
+make systemd-enable-now
+```
+
+If you only want to start certain services instead, start them like you normally would with systemd.
+
+[openssh]: https://www.archlinux.org/packages/core/x86_64/openssh/
+[rxvt-unicode-patched]: https://aur.archlinux.org/packages/rxvt-unicode-patched/
+[mpd]: https://www.archlinux.org/packages/extra/x86_64/mpd/