Mention memory usage for Elasticsearch

[vmstan]

          I found this (apperently 4 year old version of mastodon docs) quite useful:

https://github.com/martindale/mastodon-docs/blob/master/Running-Mastodon/Elasticsearch-guide.md

Note: Are you worried about how much memory configuration need on your VPS? This list may be useful! In my opinion, It would be better to set it from 35% to 50% of the total memory.

Memory	Xms	Xmx
1GB	256m	256m
2GB	512m	512m
3GB	1g	1g
4GB	2g	2g

Originally posted by @allilengyi in https://github.com/mastodon/mastodon/issues/15440#issuecomment-1307689239