Add AssetBuyerManager to README

packages/asset-buyer/README.md

@@ -12,16 +12,30 @@ yarn add @0xproject/asset-buyer
 
 **Import**
 
+For single-asset price estimation and purchase:
+
 ```typescript
 import { AssetBuyer } from '@0xproject/asset-buyer';
 ```
 
+For multiple assets:
+
+```typescript
+import { AssetBuyerManager } from '@0xproject/asset-buyer';
+```
+
 or
 
 ```javascript
 var AssetBuyer = require('@0xproject/asset-buyer').AssetBuyer;
 ```
 
+and
+
+```javascript
+var AssetBuyerManager = require('@0xproject/asset-buyer').AssetBuyerManager;
+```
+
 If your project is in [TypeScript](https://www.typescriptlang.org/), add the following to your `tsconfig.json`:
 
 ```json