doc readline configuration

[Kevin Ryde]

I added a bit to the readline docs:


   You can specify a different history file by setting the environment
variable `GUILE_HISTORY'.  And you can make Guile specific
customizations to your `.inputrc' by testing for application `Guile'
(*note Conditional Init Constructs: (readline)Conditional Init
Constructs.).  For instance to define a key inserting a matched pair of
parentheses,

     $if Guile
       "\C-o": "()\C-b"
     $endif