add reference of: using continuous distribution rather than discrete in {cfr}

[avallecam]

in https://github.com/epiverse-trace/tutorials-middle/pull/1/commits/b8eab81d08c1ff77ff1865f129ebb82bca7643b3 we added a callout on using discrete over continuous, but we can add a reference to make this visible.

follow up on https://github.com/epiverse-trace/howto/issues/37 to include them when available.