Add jemalloc as linux prereq

[alexandratran]

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Describe the change

Document jemalloc as prerequisite for installing Besu on Linux.

Issue fixed

fixes #1289

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Preview

[siladu]

Looks good but it's not a pre-requisite, it's an optional, recommended performance improvement

[siladu]

LGTM

[bgravenorst]

LGTM