build static linked binary (#60)

Add build guide for making static-linked binaries

build/Dockerfile

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+FROM ekidd/rust-musl-builder
+
+# Build a static copy of libsodium.
+RUN VERS=1.0.13 && \
+    cd /home/rust/libs && \
+    curl -LO https://download.libsodium.org/libsodium/releases/libsodium-$VERS.tar.gz && \
+    tar xzf libsodium-$VERS.tar.gz && cd libsodium-$VERS && \
+    CC=musl-gcc ./configure --prefix=/usr/local/musl && \
+    make && sudo make install && \
+    cd .. && rm -rf libsodium-$VERS.tar.gz libsodium-$VERS
+
+ENV SODIUM_STATIC=yes
+ENV SODIUM_LIB_DIR=/usr/local/musl/lib

build/README.md

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+Compile static-linked binaries with [`rust-musl-builder`](https://github.com/emk/rust-musl-builder):
+
+```sh
+# build image use Dockerfile in build dir.
+docker build -t shadowsocks_rust:0.0.1 .
+# run command in project root.
+alias rust-musl-builder='docker run --rm -it -v "$(pwd)":/home/rust/src shadowsocks_rust:0.0.1'
+rust-musl-builder cargo build --release
+```
+
+At the moment, it doesn't attempt to cache libraries between builds, so this is best reserved for making final release builds.