Updates to README

[jpolton]

Task list for updates to README

• [x] Edit livljobs JAVA_HOME variable: export JAVA_HOME=/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.372.b07-1.el7_9.x86_64/
• [X] Add Mac JAVE_HOME variable
• [x] Make note that the conda ENV set up currently does not work for ARM64 [no longer an issue as solved with #123]
• [X] Add wget instructions for benchmark data
• [X] Add explicit example on how to use benchmark data
• [X] Add explicit examples for making images with benchmark data
• [x] Fix layout of images

[jpolton]

Would it better for an empty /benchmark dir to exist in the main?

Good idea. Though I think GitHub doesn't like empty directories so in practise you have to add a README inside that says some thing like "This directory stores the benchmark data, obtainable from ..."

[jpolton]

It looks like test_scripts/ is superseded by plotting/. @jdha Is this true?

(The README references test_scripts in the plotting bit, but I think that this can be removed/replaced/tidied?)

[rdPatmore]

@jpolton, yes, I'd raised an issue on this earlier (#121). @jdha has it on the agenda for removal (#115). Mentions will also need to be removed in the README.md I guess.

[jdha]

I added a tick box to your list @jpolton about AMR64 ... hopefully once we migrate to cartopy this will help with installing on a Mac M1/2