Update readme to reflect the need for libtool package

Here is the error I got originally:
```bash
autoreconf: running: aclocal --force -I m4
autoreconf: configure.ac: tracing
autoreconf: configure.ac: not using Libtool
autoreconf: running: /usr/local/Cellar/autoconf/2.69/bin/autoconf --force
autoreconf: running: /usr/local/Cellar/autoconf/2.69/bin/autoheader --force
autoreconf: running: automake --add-missing --copy --force-missing
configure.ac:12: installing './compile'
configure.ac:8: installing './install-sh'
configure.ac:8: installing './missing'
PosixSocketClient/Makefile.am:6: error: Libtool library used but 'LIBTOOL' is undefined
PosixSocketClient/Makefile.am:6:   The usual way to define 'LIBTOOL' is to add 'LT_INIT'
PosixSocketClient/Makefile.am:6:   to 'configure.ac' and run 'aclocal' and 'autoconf' again.
PosixSocketClient/Makefile.am:6:   If 'LT_INIT' is in 'configure.ac', make sure
PosixSocketClient/Makefile.am:6:   its definition is in aclocal's search path.
PosixSocketClient/Makefile.am:8: warning: source file 'src/EClientSocketBase.cpp' is in a subdirectory,
PosixSocketClient/Makefile.am:8: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.
PosixSocketClient/Makefile.am:8: warning: source file 'src/EPosixClientSocket.cpp' is in a subdirectory,
PosixSocketClient/Makefile.am:8: but option 'subdir-objects' is disabled
PosixSocketClient/Makefile.am: installing './depcomp'
autoreconf: automake failed with exit status: 1
```