README.md: mention disabling parallel build on Pi

On bullseye it seems to default to 'make -j4' which will results
in OOM when it runs out of swap

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[ 3004.100215] Out of memory: Killed process 6983 (cc1plus) total-vm:211660kB, anon-rss:170332kB, file-rss:0kB, shmem-rss:0kB, UID:1000 pgtables:428kB oom_score_adj:0
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Mention how to disable parallel build in README.md