Add note for repository to add new wordlists

[vladimirfomene]

Many wordlists were proposed here, but they were closed with the following note:

For now, the author(s) of BIP 39 have decided not to accept any further word lists into BIP 39 itself, and encourage adding new ones to the WLIPs repo here: https://github.com/p2w34/wlips

I have added the note to the BIP's wiki so that people will know where to open these PRs going forward.