Add recipe for TypeScript plugin

doc/recipes/plugins.md

@@ -0,0 +1,71 @@
+# Plugins
+
+## TypeScript Example
+
+```ts
+export function Mixin(options = {}) {
+  return function<T extends typeof Model>(Base: T) {
+    return class extends Base {
+      mixinMethod() {}
+    };
+  };
+}
+
+// Usage
+
+class Person extends Model {}
+
+const MixinPerson = Mixin(Person);
+
+// Or as a decorator:
+
+@Mixin
+class Person extends Model {}
+```
+
+## TypeScript Example with Custom QueryBuilder
+
+```ts
+class CustomQueryBuilder<M extends Model, R = M[]> extends QueryBuilder<M, R> {
+  ArrayQueryBuilderType!: CustomQueryBuilder<M, M[]>;
+  SingleQueryBuilderType!: CustomQueryBuilder<M, M>;
+  NumberQueryBuilderType!: CustomQueryBuilder<M, number>;
+  PageQueryBuilderType!: CustomQueryBuilder<M, Page<M>>;
+
+  someCustomMethod(): this {
+    return this;
+  }
+}
+
+export function CustomQueryBuilderMixin(options = {}) {
+  return function<T extends typeof Model>(Base: T) {
+    return class extends Base {
+      static QueryBuilder = QueryBuilder;
+      QueryBuilderType: CustomQueryBuilder<this, this[]>;
+
+      mixinMethod() {}
+    };
+  };
+}
+
+// Usage
+
+class Person extends Model {}
+
+const MixinPerson = CustomQueryBuilderMixin(Person);
+
+// Or as a decorator:
+
+@CustomQueryBuilderMixin
+class Person extends Model {}
+
+async () => {
+  const z = await MixinPerson.query()
+    .whereIn('id', [1, 2])
+    .someCustomMethod()
+    .where('foo', 1)
+    .someCustomMethod();
+
+  z[0].mixinMethod();
+};
+```