Update Readme according to Repo migration

Add instruction to contribute on https://repo.getmonero.org/monero-project/monero-site as the repository has been moved.

MR created here following @fluffypony's instruction on IRC:

<fluffypony> el00ruobuob: we're (presumably) going to sync upstream from repo to github
<fluffypony> so maybe that should be merged on repo first?
<fluffypony> then we can test the mirror push