Clarify the use of the `JVM_OPTS` env var in the README's Docker instructions

[mfitz]

This section of the README includes instructions on how to use the JVM_OPTS env var to pass JVM parameters into the Gelato Docker container. We have recently seen an Alpha tester try to set multiple different JVM_OPTS env vars when trying to set more than one JVM parameter, thus:

docker run \
-v ${PWD}/monty-data/:/my-model-outputs \
-v ${PWD}/monty-data/:/gelato-out \
-e JVM_OPTS="-Xmx100G" \
-e JVM_OPTS="-XX:MaxRAMPercentage=80.0" \
gelato \
-mc /my-model-outputs/output_config.xml \
-mo /my-model-outputs \
-o /gelato-out

The instructions should make it clear that there should be only one JVM_OPTS env var, and that it can contain multiple JVM parameters in a space-separated list inside the string.