Add README to dnf plugin

plugins/dnf/README.md

+## Description
+
+This plugin makes `dnf` usage easier by adding aliases for the most
+common commands.
+
+`dnf` is the new package manager for RPM-based distributions, which
+replaces `yum`.
+
+## Aliases
+
+| Alias | Command                 | Description              |
+|-------|-------------------------|--------------------------|
+| dnfl  | `dnf list`              | List packages            |
+| dnfli | `dnf list installed`    | List installed packages  |
+| dnfgl | `dnf grouplist`         | List package groups      |
+| dnfmc | `dnf makecache`         | Generate metadata cache  |
+| dnfp  | `dnf info`              | Show package information |
+| dnfs  | `dnf search`            | Search package           |
+| **Use `sudo`**                                             |
+| dnfu  | `sudo dnf upgrade`      | Upgrade package          |
+| dnfi  | `sudo dnf install`      | Install package          |
+| dnfgi | `sudo dnf groupinstall` | Install package group    |
+| dnfr  | `sudo dnf remove`       | Remove package           |
+| dnfgr | `sudo dnf groupremove`  | Remove package group     |
+| dnfc  | `sudo dnf clean all`    | Clean cache              |