[Docs] Add the steps to build docs (#409)

docs/README.md

+## Build Documentation
+
+1. Clone MMEngine
+
+   ```bash
+   git clone https://github.com/open-mmlab/mmengine.git
+   cd mmegnine
+   ```
+
+2. Install the building dependencies of documentation
+
+   ```bash
+   pip install requirements/docs.txt
+   ```
+
+3. Change directory to `docs/en` or `docs/zh_cn`
+
+   ```bash
+   cd docs/en  # or docs/zh_cn
+   ```
+
+4. Build documentation
+
+   ```bash
+   make html
+   ```
+
+5. Open `_build/html/index.html` with browser