add usage

README.md

-# VisualDL
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+# VisualDL
+
+
+### How to use
+#### Step 1: build and install Python package
+```shell
+python setup.py bdist_wheel
+cd dist
+pip install visualdl-0.0.1-py2-none-any.whl
+```
+
+#### Step 2: build frontend
+```shell
+cd visualdl/frontend
+npm install
+npm run build
+```
+
+### Step 3: run
+```
+python bin/visual_dl.py --port=8888
+```
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