post-install tuning suggestions

we should document:

• tuning postgres (pgtune&c)
• giving redis enough RAM and making sure it's not evicting keys
• tuning the clusterLimit and concurrency values in Sharkey

added enhancement label

see also #15

we should probably link to https://blog.lattemacchiato.dev/how-to-make-your-fedi-instance-performance-not-suck/