Add Amazon affiliate links to books we've read

I've added these links to the books we've read. They were generated
using the Amazon affiliate tools. We can probably auto-generate them and
add them automatically if we move the book lists into `_data/`, but we
can do that at a later stage.

For the record, the affiliate account is administered by me, Chris
Lowis, but I commit to using any revenues to purchase books to donate to
the London Computation Club for the use of any members.

Closes #21, fixes #6.