How do I install GNU Readline?

You can install the packages by using the command in a terminal (Press Ctrl-Alt-T to bring it).

sudo apt-get install libreadline6 libreadline6-dev

The package libreadline is for running applications using readline command

and the package libreadline-dev is for compiling and building readline application.

Credit goes to this blog post

libreadline6 don't work anymore.

The latest one is lib32readline7.

Or it's better to look at the available libraries along with a short description using following code:

apt-cache search readline

sudo apt-get install lib32readline7 lib32readline-dev 

can be used.