Update README.md

Author: Celos

README.md

@@ -1,2 +1,58 @@
 # avro-json-decoder
 JSON decoder for AVRO that infers default values. Based on org.apache.avro.io.JsonDecoder from <a href="https://github.com/apache/avro">AVRO</a> 1.8.2 and org.apache.avro.io.ExtendedJsonDecoder by <a href="https://github.com/zolyfarkas/avro">zolyfarkas</a>.
+
+## why
+
+Given this schema (in AVRO IDL)
+
+```
+record User {
+  string username;
+  union {null, string} name = null;
+}
+```
+this record is valid and will be decoded properly
+```json
+{"username":"user1","name":null}
+```
+whereas this record
+```json
+{"username":"user1"}
+```
+will produce something like
+```
+org.apache.avro.AvroTypeException: Expected field name not found: name
+	at org.apache.avro.io.JsonDecoder.doAction(JsonDecoder.java:495)
+	at org.apache.avro.io.parsing.Parser.advance(Parser.java:88)
+	at org.apache.avro.io.JsonDecoder.advance(JsonDecoder.java:157)
+	at org.apache.avro.io.JsonDecoder.readIndex(JsonDecoder.java:447)
+	...
+```
+or
+```
+org.apache.avro.AvroTypeException: Expected start-union. Got END_OBJECT
+	at org.apache.avro.io.JsonDecoder.error(JsonDecoder.java:698)
+	at org.apache.avro.io.JsonDecoder.readIndex(JsonDecoder.java:441)
+	at org.apache.avro.io.ResolvingDecoder.doAction(ResolvingDecoder.java:290)
+	at org.apache.avro.io.parsing.Parser.advance(Parser.java:88)
+        ...
+```
+
+The decoder allows decoding JSON that doesn't specify optional values, provided they have defaults.
+
+## how
+
+Replace
+
+```java
+Decoder decoder = DecoderFactory.get().jsonDecoder(SCHEMA, INPUT_STREAM_OR_STRING);
+```
+with
+```java
+Decoder decoder = new ExtendedJsonDecoder(SCHEMA, INPUT_STREAM_OR_STRING);
+```
+and pass it to your reader, as usual:
+```java
+SpecificDatumReader<T> reader = new SpecificDatumReader<>(SCHEMA_OR_CLASS);
+reader.read(null, decoder);
+```