add migration

docs/migrations/v3-to-v4.md

@@ -0,0 +1,24 @@
+# Migrating from v2 to v3
+
+The ONLY thing that changes between v3 and v4 has to do with the standalone browser bundle.
+
+## New browser bundle
+
+In the old browser bundle you only had access to the parser.
+
+```js
+const parser = new window.AsyncAPIParser();
+const spec = '{ ... }';
+const { document: parsedDocument, diagnostics } = await parser.parse(spec);
+```
+
+With the new browser bundle, you have access a bunch of support functions such as `fromURL`, `convertToOldAPI`, `unstringify`, `stringify`.
+
+```js
+const parser = new window.AsyncAPIParser.Parser();
+const spec = '{ ... }';
+const { document: parsedDocument, diagnostics } = await parser.parse(spec);
+...
+const result = window.AsyncAPIParser.fromURL(parser, 'http://localhost:8080/asyncapi.json');
+const {document: parsedDocument, diagnostics} = await result.parse();
+```