Adds backend / SDK dev doc (#262)

* Adds backend / SDK dev doc

* Update per Varun and chanwei's comment

docs/how_to_dev_backend_en.md

+# VisualDL Backend / SDK structure and Develop Guide
+
+VisualDL has three components.
+1. The Python/C++ SDK that logs the data during training.
+1. The single page client side app that visualized training data.
+1. The server (powered on Flask) that reads the logs data and delivers it to the client side app for displaying graphs (scalars/histograms) and embeddings.
+
+This doc will go over Backend / SDK structure with development guide.
+
+For front end architecture and development guide, looks [here](./how_to_dev_frontend_en.md)
+
+
+## Code structure
+
+All backend and sdk logic is under visualdl sub directory
+
+```
+├── build
+├── build.sh
+├── CMakeLists.txt
+├── frontend
+├── visualdl
+    ├── logic    - API for C++ SDK, information maintainer 
+    ├── python   - API for Python SDK
+    ├── server   - Flask server, provides front end APIs for read data
+    ├── storage  - Data structure and protobuf
+    └── utils    - Helper function and classes
+```
+
+## Development
+
+Any code changes in ```server``` folder, simply run
+
+```
+python visualdl/server/visualDL --logdir={LOG_DIR} --port=8080
+```
+to restart flask server
+
+Notice if you install visualDL from pip install wheel, make sure resetting PYTHONPATH to get the command work.
+
+Any code changes in ```logic```, ```python```, ```storage```, ```utils``` is modified, need to rebuild core.so or sdk to get effect.
+
+Either run VisualDL/build.sh that builds everything or a target like this
+```
+mkdir {VisualDL_ROOT}/build
+cd build
+cmake ..
+make {TARGET MODULE}
+```
+
+
+## Tests
+
+Modification in SDK API and any major function changes should be adding test cases.
+
+
+In {VisualDL_ROOT}/CMakeList.txt, we have a list of test files as target
+
+```
+add_executable(vl_test
+        ${PROJECT_SOURCE_DIR}/visualdl/test.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/logic/sdk_test.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/logic/histogram_test.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/storage/storage_test.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/storage/test_binary_record.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/utils/test_concurrency.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/utils/test_image.cc
+        ${PROJECT_SOURCE_DIR}/visualdl/utils/concurrency.h
+        ${PROJECT_SOURCE_DIR}/visualdl/utils/filesystem.h
+        )
+```
+
+Add new test cases in corresponding files where code is modified or create new test files. You can make vl_test target and just run vl_test executable for all test cases.
+
+```
+mkdir {VisualDL_ROOT}/build
+cd build
+cmake ..
+make vl_test
+./vl_test
+```
+
+## Backend / SDK architecture
+
+### Server
+
+- Implement a server using a lightweight framework Flask, provides two services:
+  - Host a main application with support of a front end web app 
+  - Provide a series of HTTP end points, using JSON for front end data communication
+- ```visualDL``` : main app entry point
+  - defines server arguments and config
+  - provides API and router for front end
+- ```lib.py``` : delegate wrapper for function calls
+  - read and caches log data
+  - convert data to JSON
+  - retry mechanism 
+- ```graph.py``` and ```onnx```
+  - graph.py defines the logic to read and parse onnx model file
+  - create edges and nodes as JSON format for front end to read
+  - onnx folder contains onnx protobuf interface that ports from onnx repo
+
+
+### Logic & SDK
+
+#### Python SDK
+- ```pybind.cc``` : create python interface from C++ layer
+- ```storage.py``` : ```LogReader``` and ```LogWriter``` APIs of Python SDK that maps to pybind
+
+#### C++ SDK
+- ```sdk.h```
+  - defines ```LogReader``` and ```LogWriter``` APIs of C++ SDK
+  - defines feature components such as ```Scalar```, ```Histogram```, ```Image``` and their corresponding component```Reader```
+- ```sdk.cc``` : implements ```Reader``` to use ```Storage``` layer for reading and writing
+
+#### Information Container
+- Currently it just handles logic of sync cycle (when to write)
+
+#### Storage
+- Defines log data format as following:
+  - ```Storage``` : main set of data with different modes such as "train" or "test", contain multiple ```Tablet```
+  - ```Tablet``` : group a single type of data with ```tag```. One tablet will only represent one data type such as scalar, histogram, image and contains multiple ```Record```. For example one tablet represents a line in scalar chart. 
+  - ```Record``` : represent any data structure for any component
+- Handles the actual logic to serialized to disk and deserialized from disk
+- ```storage.proto```: protobuf file that defines the exact interface to be read/write