Add operationalisation section to README, guidance on jemalloc

[jebbench]

Adds some information on the memory use of libvips in long running applications and how to improve it.

[lopcode]

Thanks for the PR! Hoping to look through things over the next few days, but I'm also travelling, so could be next week.

The README does seem like a natural place to put usage tips. If we gather too many maybe it needs reorganised again, but fine to start with.

I'm unsure about changing allocators - my understanding is it can be dangerous but that could be wrong. I'd happily read some guidance if you can point me to some.

[jebbench]

You could use the github wiki I guess if the README becomes too long.

The memory thing caught me out so I thought it was worth mentioning.

The libvips "developer checklist" suggests using jemalloc: https://www.libvips.org/API/8.17/developer-checklist.html#linux-memory-allocator

I couldn't find any official advice on using jemalloc with the JVM but there are numerous blog posts detailing it's use to solve memory issues (and a couple of JVM bugs that only appear when using it).

Thank's for the library and being so responsive!

[lopcode]

@jebbench I grew the scope of this PR a bit:

• Adds a jemalloc Docker test variant
• Runs the Docker tests in parallel on GitHub Actions
• Adds a section to the README documenting "Operationalisation"
  • Includes your documentation about jemalloc and memory usage, with some of my own tweaks
  • Also includes a section on "untrusted operations"

If you're comfortable with the tweaks I made to your documentation, I'll merge it in 🙂

[jebbench]

LGTM