add readme

README.md

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+# shopware-typedef
+
+> Shopware Typescript typings
+
+```bash
+npm install --save-dev shopware-typedef
+```
+
+In your code it should probably look somethin like this:
+
+```typescript
+/// <reference types="shopware-typedef"/>
+
+export class ScnFoo extends $.PluginBase implements SwPluginDefinition {
+    init(){
+        console.log('hello, world!');
+    }
+}
+
+$.plugin('scnFoo', ScnFoo.prototype);
+
+declare global {
+    interface JQuery { scnFoo(): JQuery }
+    interface SwPluginsCollection { scnFoo: ScnFoo }
+}
+```
+
+And that's about all to do for adding a plugin. Have a look into the example.ts for more help.
+
+Also, be aware that you can't yet pass `$.plugin()` a constructor as a second argument. There's already an issue at shopware for that: [#1489](https://github.com/shopware/shopware/pull/1489)
+