Document how to apply updated scheduler templates

[n3orma]

Fixes #288

This PR clarifies that changes to the batchtools configuration file or a scheduler template referenced by it can be applied by reloading the existing registry with loadRegistry().

This avoids the need to remove and recreate the registry when updating the configuration or job template.

[jemus42]

Maybe this should rather be placed in the loadRegistry docs? And it should explain the mechanism, i.e. that conf.file is source on every load, and the cluster functions are thereby reloaded as well (which may make it easier to reg$cluster.functions = makeClusterFunctionsSlurm("new.tmpl")).

Also worth pointing out that only newly submitJobs()'d jobs pick it up, just in case there may be confusion around queued jobs or so.

[jemus42]

Cool, thanks!