Documentation: cross build of corefx for arm

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>

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+Cross Compilation for ARM on Linux
+==================================
+
+Through cross compilation, on Linux it is possible to build CoreFX for arm or arm64.
+It is almost similiar to the cross compilation procedure of CoreCLR. 
+
+Requirements
+------------
+
+You need a Debian based host and the following packages needs to be installed:
+
+    lgs@ubuntu ~/git/corefx/ $ sudo apt-get install qemu qemu-user-static binfmt-support debootstrap
+
+In addition, to cross compile CoreFX the binutils for the target are required. So for arm you need:
+
+    lgs@ubuntu ~/git/corefx/ $ sudo apt-get install binutils-arm-linux-gnueabihf
+
+and conversely for arm64:
+
+    lgs@ubuntu ~/git/corefx/ $ sudo apt-get install binutils-aarch64-linux-gnu
+
+
+Generating the rootfs
+---------------------
+The `cross\build-rootfs.sh` script can be used to download the files needed for cross compilation. It will generate an Ubuntu 14.04 rootfs as this is what CoreFX targets.
+
+    Usage: build-rootfs.sh [BuildArch]
+    BuildArch can be: arm, arm64
+
+The `build-rootfs.sh` script must be run as root as it has to make some symlinks to the system, it will by default generate the rootfs in `cross\rootfs\<BuildArch>` however this can be changed by setting the `ROOTFS_DIR` environment variable.
+
+For example, to generate an arm rootfs:
+
+    lgs@ubuntu ~/git/corefx/ $ sudo ./cross/build-rootfs.sh arm
+
+and if you wanted to generate the rootfs elsewhere:
+
+    lgs@ubuntu ~/git/corefx/ $ sudo ROOTFS_DIR=/home/lgs/corefx-cross/arm ./build-rootfs.sh arm
+
+
+Cross compiling for native CoreFX
+---------------------------------
+Once the rootfs has been generated, it will be possible to cross compile CoreFX. If `ROOTFS_DIR` was set when generating the rootfs, then it must also be set when running `build.sh`.
+
+So, without `ROOTFS_DIR`:
+
+    lgs@ubuntu ~/git/corefx/ $ ./build.sh native arm debug verbose clean cross
+
+And with:
+
+    lgs@ubuntu ~/git/corefx/ $ ROOTFS_DIR=/home/lgs/corefx-cross/arm ./build.sh native arm debug verbose clean cross
+
+As usual the generated binaries will be found in `bin/BuildOS.BuildArch.BuildType/Native` as following:
+
+    lgs@ubuntu ~/git/corefx/ ls -al ./bin/Linux.arm.Debug/Native
+    total 988
+    drwxrwxr-x 2 lgs lgs   4096  3  6 18:33 .
+    drwxrwxr-x 3 lgs lgs   4096  3  6 18:33 ..
+    -rw-r--r-- 1 lgs lgs  19797  3  6 18:33 System.IO.Compression.Native.so
+    -rw-r--r-- 1 lgs lgs 428232  3  6 18:33 System.Native.a
+    -rw-r--r-- 1 lgs lgs 228279  3  6 18:33 System.Native.so
+    -rw-r--r-- 1 lgs lgs  53089  3  6 18:33 System.Net.Http.Native.so
+    -rw-r--r-- 1 lgs lgs 266720  3  6 18:33 System.Security.Cryptography.Native.so
+    lgs@ubuntu ~/git/corefx/ file ./bin/Linux.arm.Debug/Native/System.Native.so 
+    ./bin/Linux.arm.Debug/Native/System.Native.so: 
+    ELF 32-bit LSB  shared object, ARM, EABI5 version 1 (SYSV), 
+    dynamically linked, BuildID[sha1]=fac50f1bd657c1759f0ad6cf5951511ddf252e67, not stripped
+
+
+Compiling for managed CoreFX
+----------------------------
+In order to generate managed CoreFX for ARM, we can compile CoreFX with native build because managed CoreFX is independent on BuildOS and BuildArch.
+
+    lgs@ubuntu ~/git/corefx/ $ ./build.sh managed debug clean verbose 
+
+The output is at bin/<BuildOS>.AnyCPU.Debug.
+
+
+