Add info for custom markdown parser.

MARKDOWN.md

@@ -0,0 +1,65 @@
+# Markdown Parser
+
+By default apiDoc uses [markdown-it](https://github.com/markdown-it/markdown-it) Markdown Parser.
+
+
+
+## Custom Parser
+
+`apidoc --markdown /path/to/a/custom_markdown_parser.js`
+
+Your custom parser must return a class with a render-method.
+
+
+
+## Examples
+
+### With "markdown-it" Markdown Parser.
+
+```js
+var Markdown = require('markdown-it');
+
+function CustomMarkdownParser() {
+    this.markdownParser = new Markdown({
+        breaks     : false,
+        html       : true,
+        linkify    : true,
+        typographer: false
+    });
+}
+
+module.exports = CustomMarkdownParser;
+
+CustomMarkdownParser.prototype.render = function(text) {
+    return this.markdownParser.render(text);
+};
+```
+
+
+
+### With "marked" Markdown Parser.
+
+**marked seemed to be outdated and has currently a security issue, please use it not anymore in production.**
+This is only an demo.
+
+```js
+var markdownParser = require('marked');
+
+function CustomMarkdownParser() {
+    markdownParser.setOptions({
+        gfm        : true,
+        tables     : true,
+        breaks     : false,
+        pedantic   : false,
+        sanitize   : false,
+        smartLists : false,
+        smartypants: false
+    });
+}
+
+module.exports = CustomMarkdownParser;
+
+CustomMarkdownParser.prototype.render = function(text) {
+    return markdownParser(text);
+};
+```