Add compiling instructions for android (#278)

* Add compiling instructions for android

docs/Android/Compiling.md

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+# Establishing a Build Environment
+## Installing the Android NDK
+The Android NDK is required to build native modules for Android.  
+Download the appropriate NDK archive from the following site: [Download the Android NDK](https://developer.android.com/ndk/downloads/index.html)  
+To install the Android NDK, simply expand the archive in the folder where you want to install it.
+## Creating the Toolchain
+Run ```mktoolchains``` script to prepare standalone toolchains for all supported architectures. Refer to [NDK docs](https://developer.android.com/ndk/guides/standalone_toolchain.html) for details.
+## Working with Clang and libc++
+GNU compilers and gnustl will be removed from NDK starting Q3 2018, so we will use clang and libc++.
+## OpenSSL
+Google removed openssl from Android 7+. You must build openssl libs by yourself.
+# Build SRT for Android
+Run ```/bin/bash mkall```. Libraries will be installed to ```./target-architecture/lib```.

docs/Android/mkall

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+#!/bin/bash
+
+openssl_ver=1.0.2n
+srt_branch=dev
+
+BASE_DIR=$(readlink -f $0 | xargs dirname)
+
+wget -N https://www.openssl.org/source/openssl-$openssl_ver.tar.gz
+
+if [ ! -d $BASE_DIR/srt ]; then
+ git clone https://github.com/Haivision/srt srt
+ git -C $BASE_DIR/srt checkout -b $srt_branch origin/$srt_branch
+fi
+
+declare -A target_hosts
+target_hosts=(
+ [arm]=arm-linux-androideabi
+ [arm64]=aarch64-linux-android
+ [x86]=i686-linux-android
+ [x86_64]=x86_64-linux-android
+)
+
+declare -A sysroots
+sysroots=(
+ [arm]=$BASE_DIR/armeabi-v7a
+ [arm64]=$BASE_DIR/arm64-v8a
+ [x86]=$BASE_DIR/x86
+ [x86_64]=$BASE_DIR/x86_64
+)
+
+for arch in arm arm64 x86 x86_64; do
+ rm -rf $BASE_DIR/openssl-$openssl_ver
+ tar xf $BASE_DIR/openssl-$openssl_ver.tar.gz
+ $BASE_DIR/mkssl -t $BASE_DIR/android-toolchain-$arch -h ${target_hosts[$arch]} -s $BASE_DIR/openssl-$openssl_ver -i ${sysroots[$arch]}
+
+ git -C $BASE_DIR/srt clean -fd
+ $BASE_DIR/mksrt -t $BASE_DIR/android-toolchain-$arch -h ${target_hosts[$arch]} -s $BASE_DIR/srt -i ${sysroots[$arch]}
+done

docs/Android/mksrt

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+#!/bin/bash
+
+source prepare_build
+
+./configure --CMAKE_PREFIX_PATH=$install_dir --CMAKE_INSTALL_PREFIX=$install_dir
+make
+make install

docs/Android/mkssl

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+#!/bin/bash
+
+source prepare_build
+
+./Configure android --openssldir=$install_dir
+# OpenSSL (1.0.2) adds -mandroid to the compile flags. This flag is not recognized by clang.
+sed -i 's/-mandroid//g' Makefile
+make
+make install

docs/Android/mktoolchains

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+#!/bin/bash
+
+BASE_DIR=$(readlink -f $0 | xargs dirname)
+
+export PATH=$PATH:/opt/android-ndk-r16b/build/tools
+
+for arch in arm arm64 x86 x86_64; do
+ make_standalone_toolchain.py -v --arch $arch --api 21 --stl=libc++ --install-dir=$BASE_DIR/android-toolchain-$arch
+done

docs/Android/prepare_build

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+#!/bin/bash
+
+while getopts t:h:s:i: option
+do
+ case "${option}"
+ in
+ t) toolchain_dir=${OPTARG};;
+ h) target_host=${OPTARG};;
+ s) src_dir=${OPTARG};;
+ i) install_dir=$OPTARG;;
+ esac
+done
+
+# Add the standalone toolchain to the search path.
+export PATH=$PATH:$toolchain_dir/bin
+
+# Tell configure what tools to use.
+export AR=$target_host-ar
+export AS=$target_host-clang
+export CC=$target_host-clang
+export CXX=$target_host-clang++
+export LD=$target_host-ld
+export STRIP=$target_host-strip
+
+# Tell configure what flags Android requires.
+export CFLAGS="-fPIE -fPIC"
+export LDFLAGS="-pie"
+
+cd $src_dir