call out memory requirements

[nathanloyer]

Could you update the readme to recommend minimum memory requirements? The settings in the docker image just say to use 75% of memory, but having a suggested minimum would help a lot. Our deployment was set to give it around 750MB and it would crash if you tried to load other IGs and then run a validation request. I've found that making sure your docker daemon has more than 2GB allocated to it and giving the java process a 2GB cap has been sufficient to work around the issue.

I'm now setting the memory with the java options of "-Xmx2048m -Xms1024m".

Either way, having some minimum requirements documented in the readme would be helpful. I noticed the documentation for the FHIR validator itself also doesn't have any recommendation.