Merge pull request #21 from ganta/add-readme

docs: Add README

README.md

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+# rehype-element-properties
+
+[![CI](https://github.com/ganta/rehype-element-properties/actions/workflows/ci.yaml/badge.svg)](https://github.com/ganta/rehype-element-properties/actions/workflows/ci.yaml)
+
+[rehype](https://github.com/rehypejs/rehype) plugin to replace element properties.
+
+## What is this?
+
+This package is a [unified](https://github.com/unifiedjs/unified) ([rehype](https://github.com/rehypejs/rehype)) plugin to replace element properties.
+You can customize the attributes of HTML elements that cannot be expressed in Markdown notation, and add embedded styles.
+
+## Install
+
+Install with [npm](https://docs.npmjs.com/cli):
+
+```shell
+npm install @ganta/rehype-element-properties
+```
+
+Install with [Yarn](https://yarnpkg.com/):
+
+```shell
+yarn add @ganta/rehype-element-properties
+```
+
+Install with [pnpm](https://pnpm.io/):
+
+```shell
+pnpm add @ganta/rehype-element-properties
+```
+
+## Usage
+
+This package is dual package (CommonJS / ES Modules).
+
+Import as CommonJS:
+
+```javascript
+const rehypeElementProperties = require("@ganta/rehype-element-properties");
+```
+
+Import as ES Modules:
+
+```javascript
+import rehypeElementProperties from "@ganta/rehype-element-properties";
+```
+
+This package is fully typed with TypeScript.
+It exports `Setting` type, which specifies the interface of the plugin setting. 
+
+```typescript
+import rehypeElementProperties, { Setting } from "@ganta/rehype-element-properties";
+```
+
+Use as a rehype plugin as follows:
+
+```typescript
+const processor = unified()
+  .use(parse, { fragment: true })
+  .use(rehypeElementProperties, {
+    img: [
+      {
+        name: "alt",
+        replacer: (value, properties) => value || properties?.src
+      },
+    ],
+  })
+```
+
+## API
+
+This package exports no identifiers.
+The default export is `rehypeElementProperties`.
+
+### `unified().use(rehypeElementProperties, setting)`
+
+#### `setting`
+
+Plugin setting.
+
+##### `setting.<tagName>`
+
+Describes the setting for each element that replaces the property.
+The tag name of the element is specified as a key.
+
+###### `setting.<tagName>[].name`
+
+Specifies the name of the property to be replaced.
+
+###### `setting.<tagName>[].replacer`
+
+Specifies the function that returns the value of the replaced property.
+
+The value types of a property is as follows:
+
+```typescript
+boolean | number | string | null | undefined | Array<string | number>
+```
+
+The function takes the value of the current property as an argument.
+The first argument is the value of the specified property.
+The second argument is the entire property of the specified element.
+
+If you want to remove a property, return `undefined`.
+
+## License
+
+[Apache License 2.0](LICENSE)