mobile_build_en (#594)

* mobile_build_en

* Review

doc/fluid/advanced_usage/deploy/mobile/mobile_build_en.md

+# Build Environment
+## Use docker 
+### 1. Install docker
+For the installation of docker, please refer to [official document](https://docs.docker.com/install/)
+### 2. Use docker to build environment
+First we enter into the directory of paddle-mobile and run `docker build` .
+Take Linux/Mac as an example (On windows, it is recommended to run in 'Docker Quickstart Terminal' )
+```
+$ docker build -t paddle-mobile:dev - < Dockerfile
+```
+Use `docker images` to show image we created
+```
+$ docker images
+REPOSITORY      TAG     IMAGE ID       CREATED         SIZE
+paddle-mobile   dev     33b146787711   45 hours ago    372MB
+```
+### 3. Use docker to build
+Enter into the directory of paddle-mobile and perform *docker run*
+```
+$ docker run -it --mount type=bind, source=$PWD, target=/paddle-mobile paddle-mobile:dev
+root@5affd29d4fc5:/ # cd /paddle-mobile
+# Generate Makefile in the construction of android
+root@5affd29d4fc5:/ # rm CMakeCache.txt
+root@5affd29d4fc5:/ # cmake -DCMAKE_TOOLCHAIN_FILE=tools/toolchains/arm-android-neon.cmake
+# Generate Makefile in the construction of linux
+root@5affd29d4fc5:/ # rm CMakeCache.txt
+root@5affd29d4fc5:/ # cmake -DCMAKE_TOOLCHAIN_FILE=tools/toolchains/arm-linux-gnueabi.cmake
+```
+### 4. Configure compiling options
+We can configure compiling options with ccmake.
+```
+root@5affd29d4fc5:/ # ccmake .
+                                                     Page 1 of 1
+ CMAKE_ASM_FLAGS
+ CMAKE_ASM_FLAGS_DEBUG
+ CMAKE_ASM_FLAGS_RELEASE
+ CMAKE_BUILD_TYPE
+ CMAKE_INSTALL_PREFIX             /usr/local
+ CMAKE_TOOLCHAIN_FILE             /paddle-mobile/tools/toolchains/arm-android-neon.cmake
+ CPU                              ON
+ DEBUGING                         ON
+ FPGA                             OFF
+ LOG_PROFILE                      ON
+ MALI_GPU                         OFF
+ NET                              googlenet
+ USE_EXCEPTION                    ON
+ USE_OPENMP                       OFF
+```
+After updating options, we can update Makefile pressing `c`, `g` .
+### 5. Build
+Use command *make* to build
+```
+root@5affd29d4fc5:/ # make
+```
+### 6. Check Output of Building
+Output can be checked on the host machine. In the directory of paddle-mobile, build and test/build, you can use command adb or scp to make it run on device.
+
+## Without docker
+Without docker, you can directly use cmake to generate makefile and then build. It needs to appropriately configure NDK_ROOT to build android with ndk. To build linux applications needs to install arm-linux-gnueabi-gcc or similiar cross-building tools and may need to configure environment variables like CC, CXX; or update arm-linux-gnueabi.cmake in tools/toolchains/ ; or add toolchain file if it is needed.