README does not mention that rsync is needed for DB download

[MamfTheKramf]

Currently the section about downloading the genetic databases does not tell you, you need rsync installed to download them. It only mentions aria2c:

https://github.com/google-deepmind/alphafold/blob/f251de6613cb478207c732bf9627b1e853c99c2f/README.md?plain=1#L62-L82

But the download_pdb_mmcif.sh script does need rsync.

As far as I can see, rsync is the only other dependency needed apart from aria2. But not mentioning it in this section or checking it at the very beginning of the download_all_data.sh script an be quite annoying when you set up the download over night and it crashes at this step with ~180GB still to download.

When I find time today, I can also submit a PR, as this are only very small changes.