docs: Add a Readme document for the adapter.

readme.txt

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+=== Asciicast Player ===
+Contributors: lamgc
+Tags: asciicast, asciiplayer, asciinema
+Requires at least: 5.2
+Tested up to: 6.2
+Stable tag: 0.1.0
+Requires PHP: 7.4
+License: GPLv3
+License URI: https://www.gnu.org/licenses/gpl-3.0.html
+
+Asciinema-Player for WordPress.
+
+== Description ==
+
+This plugin helps you insert Asciicast files into the post, better presenting terminal operations to readers.
+
+## Quick start
+
+Add a shortcode to the post and enter the following:
+
+```
+[asciiplayer]https://asciinema.org/a/28307.cast[/asciiplayer]
+```
+
+The content of the shortcode is the URL of the ascicast file.
+
+### Options
+
+The AsciiPlayer shortcode supports the options supported by Asciinema-player, such as setting `fit` to `height` and closing the control bar:
+
+```
+[asciiplayer fit="height" controls="false"]https://asciinema.org/a/28307.cast[/asciiplayer]
+```
+
+You can find all available options [here](https://www.npmjs.com/package/asciinema-player#options).
+
+> Currently, only the `logger` option is not supported because it requires an object to be passed in.
+
+#### Style options
+
+For the convenience of customizing player elements,
+the plugin has added an additional `style` **option**,
+which corresponds to the `style` **attribute** of the player container element:
+
+```
+[asciiplayer style="width: 80%;"]https://asciinema.org/a/28307.cast[/asciiplayer]
+```
+
+== Changelog ==
+
+= 0.1.0 =
+* The first available version.