Document fork (apache#413)

FORK.md

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+# Difference with upstream
+
+* SPARK-21195 - Automatically register new metrics from sources and wire default registry 
+* SPARK-20952 - ParquetFileFormat should forward TaskContext to its forkjoinpool
+* SPARK-20001 (SPARK-13587) - Support PythonRunner executing inside a Conda env (and R)
+* SPARK-17059 - Allow FileFormat to specify partition pruning strategy via splits
+* SPARK-24345 - Improve ParseError stop location when offending symbol is a token
+* SPARK-23795 - Make AbstractLauncher#self() protected 
+* SPARK-23153 - Support application dependencies in submission client's local file system
+* SPARK-18079 - CollectLimitExec.executeToIterator should perform per-partition limits
+
+* SPARK-15777 (Partial fix) - Catalog federation
+ * make ExternalCatalog configurable beyond in memory and hive
+ * FileIndex for catalog tables is provided by external catalog instead of using default impl
+
+* Better pushdown for IN expressions in parquet via UserDefinedPredicate (SPARK-17091 for original issue)
+
+# Added
+
+* Gradle plugin to easily create custom docker images for use with k8s