[Module::Build] Document install_base_relpaths

[David W. <da...@ki...>]

Module::Builders,

I just discovered install-base_relpaths, and for setting up new  
elements to simply be installed relative to whatever the install_base  
happens to be, it's very convenient. I'd like to see it documented in  
the cookbook. The attached patch adds a note about it there. Think  
you might let this be documented?

Thanks!

David