extract: add README

plugins/extract/README.md

+# extract plugin
+
+This plugin defines a function called `extract` that extracts the archive file
+you pass it, and it supports a wide variety of archive filetypes.
+
+This way you don't have to know what specific command extracts a file, you just
+do `extract <filename>` and the function takes care of the rest.
+
+To use it, add `extract` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... extract)
+```
+
+## Supported file extensions
+
+| Extension         | Description                          |
+|:------------------|:-------------------------------------|
+| `7z`              | 7zip file                            |
+| `Z`               | Z archive (LZW)                      |
+| `apk`             | Android app file                     |
+| `bz2`             | Bzip2 file                           |
+| `deb`             | Debian package                       |
+| `gz`              | Gzip file                            |
+| `ipsw`            | iOS firmware file                    |
+| `jar`             | Java Archive                         |
+| `lzma`            | LZMA archive                         |
+| `rar`             | WinRAR archive                       |
+| `sublime-package` | Sublime Text package                 |
+| `tar`             | Tarball                              |
+| `tar.bz2`         | Tarball with bzip2 compression       |
+| `tar.gz`          | Tarball with gzip compression        |
+| `tar.xz`          | Tarball with lzma2 compression       |
+| `tar.zma`         | Tarball with lzma compression        |
+| `tbz`             | Tarball with bzip compression        |
+| `tbz2`            | Tarball with bzip2 compression       |
+| `tgz`             | Tarball with gzip compression        |
+| `tlz`             | Tarball with lzma compression        |
+| `txz`             | Tarball with lzma2 compression       |
+| `war`             | Web Application archive (Java-based) |
+| `xpi`             | Mozilla XPI module file              |
+| `xz`              | LZMA2 archive                        |
+| `zip`             | Zip archive                          |
+
+See [list of archive formats](https://en.wikipedia.org/wiki/List_of_archive_formats) for
+more information regarding archive formats.