Document how to get reproducible results

[gertvv]

Originally posted by @mmkamso in https://github.com/gertvv/gemtc/issues/45#issuecomment-428729507

[gertvv]

The seeds for the JAGS RNGs and the R RNG are independent, so both need to be set:

• Use set.seed() prior to running mtc.model() to control the generation of random initial values.
• Set .RNG.name and .RNG.seed in model$inits to control the JAGS RNGs.