Update documentation with ZeroMQ information.

(cherry picked from commit 44147e5)

Author: pmienk

README.md

@@ -74,10 +74,27 @@ Next install the [build system](http://wikipedia.org/wiki/GNU_build_system):
 ```sh
 $ sudo apt-get install build-essential autoconf automake libtool pkg-config
 ```
-Next install the [Boost](http://www.boost.org) (minimum 1.57.0) development package:
+Next install the [Boost](http://www.boost.org) (1.57.0 or newer) development package:
 ```sh
 $ sudo apt-get install libboost-all-dev
 ```
+Next install the [ZeroMQ](http://www.zeromq.org) (4.2.0 or newer) development package.
+```sh
+$ sudo apt-get install libzmq5
+```
+If no satisfying libzmq5 package exists in your chosen environment, the compilation can be performed with the following instructions:
+```sh
+$ mkdir build-zeromq-4.2.0
+$ pushd build-zeromq-4.2.0
+$ wget https://github.com/zeromq/libzmq/releases/download/v4.2.0/zeromq-4.2.0.tar.gz
+$ tar --extract --file zeromq-4.2.0.tar.gz --gz --strip-components=1
+$ ./autogen.sh
+$ ./configure
+$ make
+$ sudo make install
+$ sudo ldconfig
+$ popd
+```
 Next download the [install script](https://github.com/libbitcoin/libbitcoin-server/blob/version3/install.sh) and enable execution:
 ```sh
 $ wget https://raw.githubusercontent.com/libbitcoin/libbitcoin-server/version3/install.sh
@@ -124,6 +141,10 @@ Next install the [Boost](http://www.boost.org) (1.57.0 or newer) development pac
 ```sh
 $ brew install boost
 ```
+Next install the [ZeroMQ](http://www.zeromq.org) (4.2.0 or newer) development package:
+```sh
+$ brew install zeromq
+```
 Next download the [install script](https://github.com/libbitcoin/libbitcoin-server/blob/version3/install.sh) and enable execution:
 ```sh
 $ wget https://raw.githubusercontent.com/libbitcoin/libbitcoin-server/version3/install.sh
@@ -158,6 +179,18 @@ Next install the [Boost](http://www.boost.org) (1.57.0 or newer) development pac
 ```sh
 $ sudo port install boost -no_single -no_static -python27
 ```
+Next install the [ZeroMQ](http://www.zeromq.org) (4.2.0 or newer) development package. This package does not appear to be available via MacPorts, and so follows compilation instructions:
+```sh
+$ mkdir build-zeromq-4.2.0
+$ pushd build-zeromq-4.2.0
+$ wget https://github.com/zeromq/libzmq/releases/download/v4.2.0/zeromq-4.2.0.tar.gz
+$ tar --extract --file zeromq-4.2.0.tar.gz --gz --strip-components=1
+$ ./autogen.sh
+$ ./configure
+$ make
+$ sudo make install
+$ popd
+```
 Next download the [install script](https://github.com/libbitcoin/libbitcoin-server/blob/version3/install.sh) and enable execution:
 ```sh
 $ wget https://raw.githubusercontent.com/libbitcoin/libbitcoin-server/version3/install.sh
@@ -200,13 +233,17 @@ Building and linking with a private copy of the Boost dependency:
 ```sh
 $ ./install.sh --build-boost --prefix=/home/me/myprefix
 ```
+Building and linking with a private copy of the ZeroMQ dependency:
+```sh
+$ ./install.sh --build-zmq --prefix=/home/me/myprefix
+```
 Building a statically-linked executable:
 ```sh
-$ ./install.sh --disable-shared --build-boost --prefix=/home/me/myprefix
+$ ./install.sh --disable-shared --build-boost --build-zmq --prefix=/home/me/myprefix
 ```
 Building a small statically-linked executable most quickly:
 ```sh
-$ ./install.sh CXXFLAGS="-Os -s" --without-tests --disable-shared --build-boost --prefix=/home/me/myprefix
+$ ./install.sh CXXFLAGS="-Os -s" --without-tests --disable-shared --build-boost --build-zmq --prefix=/home/me/myprefix
 ```
 Building with bash-completion support: