When I've done this sort of thing in the past - moved code into a revision control system after already having released several versions - I've made the first few commits represent the released versions. From time to time it can be really helpful to use the revision control system's capabilities to look at when a particular line of code was first introduced, eg using git blame. Be careful if you do this to check for new files - it is easy to forget to add them.

Whether it is also worth making the packaged-up releases of previous versions available via github I don't know - traditionally people would get such things from CPAN or BackPAN, but perhaps there is now a new generation of users more familiar with github than with CPAN that might make use of such things.

It's worth having a look at gitpan. It has not been updated recently so does not track your module, but it should be possible to adapt the code.