add fetch instructions to README (sql-js#436)

* add fetch instructions to README

It took me a little while to figure out, even though it's not simple, so I thought it would be a good addition to the docs

* use promise.all instead of sequential awaits

per discussion on sql-js#436

Author: llimllib

README.md

@@ -139,6 +139,19 @@ See : https://sql-js.github.io/sql.js/examples/GUI/gui.js
 
 #### Loading a database from a server
 
+##### using fetch
+
+```javascript
+const sqlPromise = initSqlJs({
+  locateFile: file => `https://path/to/your/dist/folder/dist/${file}`
+});
+const dataPromise = fetch("/path/to/databse.sqlite").then(res => res.arrayBuffer());
+const [SQL, buf] = await Promise.all([sqlPromise, dataPromise])
+const db = new SQL.Database(new Uint8Array(buf));
+```
+
+##### using XMLHttpRequest
+
 ```javascript
 var xhr = new XMLHttpRequest();
 // For example: https://github.com/lerocha/chinook-database/raw/master/ChinookDatabase/DataSources/Chinook_Sqlite.sqlite