Update Change Log for new metafounders and alternative allele frequencies functionality.

[RosCraddock]

Linked to #152

[XingerTang]

The current change log is on the branch feat_recomb_new_changelog, which is on the branch for the functionalities related to recombination rate.
To add the change log for metafounders and alternative allele frequencies, we can either:

• wait until the recomb functionalities are done and then update the documentation on the merged branch for all the newly developed functionalities, but we don't know how long it would take
• make independent change log files, but this may lead to duplication works and also possible conflicts while merging everything together at the later stage