Add GNU readline to extlibs and provided extlibs tarball

[Michael Wood-Vasey]

Here are some simplified instructions for installing GNU readline on
Mac OS X and other machines that might default to BSD readline, since
the two readlines have some incompatibilities. The suggestion here
is just to add readline to the extlibs and installing it using the
standard psconfigure etc. Besides organizational convenience, this
means that the GNU libreadline will get picked up before the system
libreadline without any additional manual intervention.

[from the appropriate directory that has 'src/ipp-2.2/']

cd src/extlibs
curl -O ​http://ftp.gnu.org/pub/gnu/readline/readline-5.1.tar.gz
tar xvzf readline-5.1.tar.gz
cd readline-5.1
psconfigure
make
make install

So my suggestion is to add the package available at

​http://ftp.gnu.org/pub/gnu/readline/readline-5.1.tar.gz

to the extlibs tarball and add in the instructions for people to
install this if needed.

[eugene]

i'll do that, but i also want to check on the APIs we are using

[eugene]

added readline to published tarball.