Amended `git submodule` instructions in the README (fixes #220).

For a fresh checkout, the README's instructions for `git submodule update` don't suffice (it does nothing at all). One must first perform an `git submodule init` before the `git submodule update`, as follows:

    $ git submodule init
    Submodule 'deps/gmock' (git://github.com/cpp-netlib/gmock) registered for path 'deps/gmock'
    Submodule 'deps/gtest' (git://github.com/cpp-netlib/gtest) registered for path 'deps/gtest'
    Submodule 'uri' (git://github.com/cpp-netlib/uri) registered for path 'uri'

    $ git submodule update
    Cloning into 'deps/gmock'...
    ...
    Submodule path 'deps/gmock': checked out '6b1759c3816d574bddde3e1725c51a811c8870e7'
    Cloning into 'deps/gtest'...
    ...
    Submodule path 'deps/gtest': checked out 'a6772271f71672e889776bfe49ec4efd9da036df'
    Cloning into 'uri'...
    ...
    Submodule path 'uri': checked out 'e3ed456dbec947765018b1dde9ede787bad91814'

(This is with Git version 1.8.0.2.)

Please amend the README instructions to mention `git submodule init`.