update readme

add usage

 # umi-blocks
 
 Official collection of some high quality blocks.
+
+## Usage
+
+```shell
+$ umi block list
+$ umi block add [A block name in list, or any github address of a umi block]
+```
+
+You can visit umi [doc](https://umijs.org/guide/block.html) for find out more info about umi block.
+
+## Contribute
+
+You can create a umi block and make a PR for us!
+
+How to create a block:
+
+```shell
+$ yarn create umi --block
+```