chore: write readme

README.md

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+# pino-pg
+A Pino Transport for PostgreSQL databases.
+
+## Installation
+```
+npm install pino-pg
+```
+
+## Usage
+Write logs into PostgreSQL
+```
+node my-app.js | pino-pg -c configuration.json
+```
+
+## Configuration file
+The configuration file is a JSON file like this :
+```json
+{
+    "connection": {
+        "user": "postgres",
+        "database": "test-database",
+        "password": "",
+        "port": "",
+        "host": "127.0.0.1"
+    },
+    "table": "logs",
+    "columns": {
+        "level": "level",
+        "time": "time",
+        "pid": "pid",
+        "hostname": "hostname",
+        "msg": "msg",
+        "v": "v"
+    }
+}
+```
+The `connection` field contains the [node-pg connection information](https://node-postgres.com/features/connecting#Programmatic).
+The `table` field is the table where you want to store logs.
+The `columns` field allows you to custom PostgreSQL column name for each pino field. Key is pino field, value is column name.
+
+## License
+Licensed under [MIT](https://github.com/Xstoudi/pino-pg/blob/master/LICENSE.md)