Adds README.md for Thrift/Avro code generation

Author: liancheng

sql/core/src/test/README.md

@@ -0,0 +1,33 @@
+# Notes for Parquet compatibility tests
+
+The following directories and files are used for Parquet compatibility tests:
+
+```
+.
+├── README.md                   # This file
+├── avro
+│   ├── parquet-compat.avdl     # Testing Avro IDL
+│   └── parquet-compat.avpr     # !! NO TOUCH !! Protocol file generated from parquet-compat.avdl
+├── gen-java                    # !! NO TOUCH !! Generated Java code
+├── scripts
+│   └── gen-code.sh             # Script used to generate Java code for Thrift and Avro
+└── thrift
+    └── parquet-compat.thrift   # Testing Thrift schema
+```
+
+Generated Java code are used in the following test suites:
+
+- `org.apache.spark.sql.parquet.ParquetAvroCompatibilitySuite`
+- `org.apache.spark.sql.parquet.ParquetThriftCompatibilitySuite`
+
+To avoid code generation during build time, Java code generated from testing Thrift schema and Avro IDL are also checked in.
+
+When updating the testing Thrift schema and Avro IDL, please run `gen-code.sh` to update all the generated Java code.
+
+## Prerequisites
+
+Please ensure `avro-tools` and `thrift` are installed.  You may install these two on Mac OS X via:
+
+```bash
+$ brew install thrift avro-tools
+```