Adding a primer for the standard module Random.

This primer intentionally does not include any functionality with Private in
the name.  It also does not include the SeedGenerators record, as that record
only includes our default random generator settings.
Add indication of the new primer to the READMEs
Taking Vass's comments into account.
Merge pull request #449 from lydia-duncan/primers

Adding a primer for the standard module Random [reviewed by vass]

This primer intentionally does not include any functionality with Private in
the name. It also does not include the SeedGenerators record, as that record
only includes our default random generator settings.