Merge pull request #103 from velconia/develop

Add python3 build script

doc/fluid/advanced_usage/development/contribute_to_paddle.md

@@ -92,6 +92,12 @@ no changes added to commit (use "git add" and/or "git commit -a")
 ➜  docker run -v $(pwd):/paddle -e "WITH_GPU=OFF" -e "WITH_AVX=ON" -e "WITH_TESTING=ON" paddle:latest-dev
 ```
 
+如果你需要在此基础上编译基于Python3的PaddlePaddle，可以：
+
+```bash
+➜  docker run -v $(pwd):/paddle -e "PY_VERSION=3.5" -e "WITH_FLUID_ONLY=ON" -e "WITH_GPU=OFF" -e "WITH_AVX=ON" -e "WITH_TESTING=ON" paddle:latest-dev
+```
+
 这个过程除了编译PaddlePaddle为 `./build/libpaddle.so`，并且输出一个 `./build/paddle.deb`文件之外，还会输出一个 `build/Dockerfile`。我们只需要运行下面命令把编译好的PaddlePaddle打包成一个*生产镜像*（`paddle:prod`）：
 
 ```bash