add mapping guidance (intel#4555)

MappingGuidance.md

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+## Mapping Guidance
+
+This document is used to provide guidance of how BigDL and Analytics zoo code are mapping to BigDL2.0
+
+* **BigDL**
+
+   ***scala***
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/XYZ``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/XYZ```
+
+    except the following:
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/dataset``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/feature/dataset```
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/transform``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/feature/transform```
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/parameters``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/optim/parameters```
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/python``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/utils/python```
+
+   ```spark/dl/src/main/scala/org/apache/spark``` to ```scala/dllib/src/main/scala/org/apache/spark```
+
+   ```spark/spark-version``` to ```scala/common/spark-version```
+
+   ```spark/dl/src/main/scala/com/intel/analytics/bigdl/nn/keras``` to ```scala/dllib/src/main/scala/com/intel/analytics/bigdl/dllib/keras```
+
+   ***python***
+
+    ```pyspark/bigdl/XYZ``` to ```python/dllib/src/bigdl/dllib/XYZ```
+
+    except the following:
+
+   ```pyspark/bigdl/dataset``` to ```python/dllib/src/bigdl/dllib/feature/dataset```
+
+   ```pyspark/bigdl/transform``` to ```python/dllib/src/bigdl/dllib/feature/transform```
+
+* **Analytics Zoo**