Add "guiding principles" section in readme about how we designed the benchmark

[holly-cummins]

Some of these are

• We wanted to prioritise realism over optimising every last bit of performance
• We wanted to show best practices in the applications
• We wanted to maintain strict parity between the Spring and Quarkus versions of the application

I've basically just drafted it so I could have done a PR, but I was trying to track and defer work. :)