[doc] Adding memory to the docker instance

[bjdmeest]

To add more memory to the docker instance, you could use the following syntax:

docker run \
    --rm -v .:/data \
    --entrypoint java \
    rmlio/rmlmapper-java:latest \
    -Xms8g -jar /rmlmapper.jar \
    --mappingfile mapping.ttl

This should be added to the README