add OOM-handling to installation instructions

[damian0604]

when running too large analyses (e.g., too large soft cosine matrix), then the OOM-killer on Ubuntu kills the ES-database (instead of, e.g., the process causing the error).
Add instructions of how to avoid that, most likely by adding the setting

[service]
OOMScoreAdjust=-500

to
/usr/lib/systemd/system/elasticsearch.service