test: add doc for testing gorush

tests/README.md

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+# Testing
+
+How to test gorush with http request?
+
+## download bat tool
+
+Download [cURL-like tool for humans](https://github.com/astaxie/bat).
+
+## testing
+
+see the JSON format:
+
+```json
+{
+  "notifications": [
+    {
+      "tokens": ["token_a", "token_b"],
+      "platform": 1,
+      "message": "Hello World iOS!"
+    },
+    {
+      "tokens": ["token_a", "token_b"],
+      "platform": 2,
+      "message": "Hello World Android!"
+    }
+  ]
+}
+```
+
+run the following command.
+
+```sh
+$ bat POST localhost:8088/api/push < tests/test.json
+```

tests/test.json

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+{
+  "notifications": [
+    {
+      "tokens": ["token_a", "token_b"],
+      "platform": 1,
+      "message": "Hello World iOS!"
+    },
+    {
+      "tokens": ["token_a", "token_b"],
+      "platform": 2,
+      "message": "Hello World Android!"
+    }
+  ]
+}