init demo material

+{
+  "name": "@umi-material/demo",
+  "version": "1.0.0",
+  "description": "a demo material of umi",
+  "main": "src/index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/umijs/umi-materials/demo"
+  },
+  "author": "yutingzhao1991 <yutingzhao1991@sina.com>",
+  "license": "ISC"
+}

+export default () => {
+  return <div>I am a demo material of umi.</div>
+}