suggestion to improve README: indicate vm.max_map_count requirement

[Sinan81]

I think it would be useful if in README, users are told to check the value of this parameter, and if necessary set it to the correct value (temporarily or permanently). Or else, datanode doesn't work properly, and preflight config fails (can't find datanode).

[boosty]

Hi @Sinan81,

the README already contains this hint:

Make sure to configure your OS or Docker to have the correct vm.max_map_count setting, see: OpenSearch Docs