Provide both remark and rehype plugins

.prettierignore

@@ -0,0 +1,2 @@
+/src/replacements/*/*.html
+/src/replacements/*/*.json

README.md

@@ -1,24 +1,80 @@
-[![codecov](https://codecov.io/gh/nasa-gcn/remark-astro/branch/main/graph/badge.svg?token=3ID7X7XNNQ)](https://codecov.io/gh/nasa-gcn/remark-astro)
+[![codecov](https://codecov.io/gh/nasa-gcn/remark-rehype-astro/branch/main/graph/badge.svg?token=3ID7X7XNNQ)](https://codecov.io/gh/nasa-gcn/remark-rehype-astro)
 
-# remark-astro
+# remark-rehype-astro
 
-This is a plugin for [remark](https://github.com/remarkjs/remark) for parsing Astro Flavored Markdown, a dialect of [Markdown](https://www.markdownguide.org) for rapid astronomy communications.
+This package provides plugins for [remark](https://github.com/remarkjs/remark) and [rehype](https://github.com/rehypejs/rehype) for parsing Astro Flavored Markdown, a dialect of [Markdown](https://www.markdownguide.org) for rapid astronomy communications. Astro Flavored Markdown detects dates, times, sky coordinates, and bibliographic references.
 
-Astro Flavored Markdown detects dates, times, sky coordinates, and bibliographic references. Astro Flavored Markdown data is tagged in the [mdast](https://github.com/syntax-tree/mdast) syntax tree for later enrichment.
+The `remark-rehype-astro` package can be used as _either_ a Remark plugin _or_ as a Rehype plugin. Use it in Remark mode if you want a symbolic representation of the Markdown content (for example, to extract data nodes from it). Use it in Rehype mode if you want to render Astro Flavored Mardkown into a Web page.
 
-An Astro Flavored Markdown node is just an mdast [Text](https://github.com/syntax-tree/mdast#text) node containing the original text verbatim plus an added `data` attribute:
+For supported syntax, see the [src/replacements](src/replacements) directory. In each directory there is a file called `test.md` illustrating the Markdown syntax, a file called `test.json` containing the resulting astract syntax tree, and a file called `test.html` showing how it is rendered in HTML.
 
-```ts
-export interface AstroText extends Text {
-  data: {
-    astro: {
-      /** Astro Flavored Markdown data type */
-      type: string
-      /** Normalized value */
-      value: string
+## Remark mode
+
+When you use the plugin in Remark mode, it tags Astro Flavored Markdown nodes in the [mdast](https://github.com/syntax-tree/mdast) syntax tree by adding `data` attributes with `class` and `value` attributes indicating the kind of data and a normalized, computable value.
+
+Here is a basic pipeline for processing in Remark mode:
+
+```mjs
+import remarkParse from 'remark-parse'
+import { remarkAstro } from 'remark-rehype-astro'
+import { unified } from 'unified'
+
+const processor = unified().use(remarkParse).use(remarkAstro)
+```
+
+The following Markdown text:
+
+```md
+See GCN Circular 123
+```
+
+is represented by the following syntax tree:
+
+```json
+{
+  "type": "root",
+  "children": [
+    {
+      "type": "text",
+      "value": "See GCN Circular "
+    },
+    {
+      "type": "text",
+      "value": "123",
+      "data": {
+        "class": "gcn-circular",
+        "value": 123
+      }
     }
-  }
+  ]
 }
 ```
 
-For supported syntax, see the [src/replacements](src/replacements) directory. In each direcetory there is a file called `test.md` illustrating the Markdown syntax and a file called `test.json` containing the resulting syntax tree.
+## Rehype mode
+
+When you use the plugin in Rehype mode, it converts Astro Flavored Markdown nodes into HTML [`<data>`](https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes) elements with `class` and `value` attributes indicating the kind of data and a normalized, computable value.
+
+Here is a basic pipeline for processing in Remark mode:
+
+```mjs
+import remarkParse from 'remark-parse'
+import remarkRehype from 'remark-rehype'
+import { rehypeAstro } from 'remark-rehype-astro'
+import { unified } from 'unified'
+
+const processor = unified().use(remarkParse).use(remarkRehype).use(rehypeAstro)
+```
+
+The following Markdown text:
+
+```md
+See GCN Circular 123
+```
+
+is represented by the following HTML:
+
+```html
+<p>See <data class="gcn-circular" value="123">123</data></p>
+```
+
+You can enrich these [`<data>`](https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes) elements using CSS, JavaScript, or by rendering them as custom React components using [rehype-react](https://github.com/rehypejs/rehype-react).