docs: note that simd-json doesn't work on big endian platforms

[jqnatividad]

In my project - qsv, I had to put in conditional compilation directives to use serde_json in big endian platforms (e.g. s390x) and only use simd-json, on little endian.

Perhaps, a short note in the README.md?

[Licenser]

This is not quite correct, I'm running simd_json on little endian (arm) systems both in dev and production. Could you share some details like failing cases, errors, tetss?

[jqnatividad]

Yep, that's what I'm reporting - simd_json works like a charm on little endian platforms.

However, on the s390x IBM Z mainframe (s390x-unknown-linux-gnu) which is big endian, it doesn't and I had to use serde_json instead with conditional compilation directives.

Check out the CI test runs on -
https://github.com/dathere/qsv/actions/workflows/rust-s390x.yml

And these are the changes I needed to make for the s390x CI to work:
dathere/qsv@d667aea
dathere/qsv@f74290e
dathere/qsv@acf2acb
dathere/qsv@f271136