docs: readme of @vuepress/markdown

packages/@vuepress/markdown/README.md

 # @vuepress/markdown
 
-> markdown for vuepress
+> markdown library for vuepress
 
-## Usage
+## Public API
 
-```javascript
-const createMarkdown = require('@vuepress/markdown')
-const md = createMarkdown(/* options */)
-```
-
-## Options
+### PLUGINS
 
-### slugify
+A map [constant](./lib/constant.js) containing the names of all built-in markdown-it plugins.
 
-- TODO
+### isRequiredPlugin(pluginName: string)
 
-### externalLinks
+- **Usage**:
 
-- TODO
+```js
+const { isRequiredPlugin } = require('@vuepress/markdown')
+console.log(isRequiredPlugin(PLUGINS.COMPONENT)) // true
+console.log(isRequiredPlugin(PLUGINS.HIGHLIGHT_LINES)) // false
+```
 
-### anchor
+### removePlugin(config: chainMarkdown, pluginName: string)
 
-- TODO
+Remove the specified built-in markdown-it plugin in VuePress.
 
-### toc
+It's needed to use with VuePress's [Plugin API > chainMarkdown](https://vuepress.vuejs.org/plugin/option-api.html#chainmarkdown).
 
-- TODO
+- **Usage**:
 
-### lineNumbers
+```js
+// You VuePress Plugin or site config.
+const { removePlugin } = require('@vuepress/markdown')
+module.exports = {
+  chainMarkdown (config) {
+    removePlugin(config, PLUGINS.HIGHLIGHT_LINES)
+  }
+}
+```
 
-- TODO
+> Note that `PLUGINS.COMPONENT` and `PLUGINS.ANCHOR` are required in VuePress, It is forbidden to delete them!
 
-### beforeInstantiate
+### removeAllBuiltInPlugins(config: chainMarkdown)
 
-- TODO
+Remove all built-in but not 100% necessary markdown-it plugins in VuePress.
 
-### afterInstantiate
+- **Usage**:
 
-- TODO
+```js
+// You VuePress Plugin or site config.
+module.exports = {
+  chainMarkdown (config) {
+    require('@vuepress/markdown').removeAllBuiltInPlugins(config)
+  }
+}
+```