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:

alphafold/README.md

Lines 62 to 82 in f251de6

	1. Download genetic databases and model parameters:
	* Install `aria2c`. On most Linux distributions it is available via the
	package manager as the `aria2` package (on Debian-based distributions this
	can be installed by running `sudo apt install aria2`).
	* Please use the script `scripts/download_all_data.sh` to download
	and set up full databases. This may take substantial time (download size is
	556 GB), so we recommend running this script in the background:
	```bash
	scripts/download_all_data.sh <DOWNLOAD_DIR> > download.log 2> download_all.log &
	```
	* **Note: The download directory `<DOWNLOAD_DIR>` should *not* be a
	subdirectory in the AlphaFold repository directory.** If it is, the Docker
	build will be slow as the large databases will be copied into the docker
	build context.
	* It is possible to run AlphaFold with reduced databases; please refer to
	the [complete documentation](#genetic-databases).

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.