Arm backend: Update backends/arm/README.md (#14206)

* Rewrite to make it more concise

Change-Id: I57c5dbc8f87a4e8f527597ec721d9697f41e5d3b


cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

Signed-off-by: Tom Allsop <[email protected]>

Author: tom-arm