Add slightly more detail about StringRecord in examples

[multimeric]

Closes #241.

One concern I have after looking at the code is how much duplication there is wrt the examples. This one example seems to have been in src/lib.rs, README.md, and also src/cookbook.rs. There's that neat script for pulling out the examples into files, but that doesn't help with these 3 cases. I wonder if the new #[doc(include="foo")] will help here?