adding doc field

```
val str =
"""
|{
| "type": "record",
| "name": "TestRecord",
| "namespace": "org.apache.avro.test",
| "doc": "test schema",
| "fields": [
| {
| "name": "name",
| "type": {
| "type": "string"
| },
| "doc": "this is name"
| },
| {
| "name": "age",
| "type": {
| "type": "long"
| },
| "doc": "this is age"
| }
| ]
|}
|""".stripMargin
```

It would be good to add a " doc " field for each data field. I don' think this feature is supported yet but happy to work with someone from the team on implementing this.