Add repository instructions to openSUSE "install" page

Since RethinkDB currently does not provide a dedicated repository for RPM-based distributions using Zypper as an interface to RPM (like openSUSE and SLE), I tried to install RethinkDB from the YUM repository for CentOS and was excited to find out that Zypper can actually handle it and install RethinkDB off of that source.

Thus, I suggest the following changes to the "Install RethinkDB on openSUSE" page of the documentation (here the file 0-getting-started/install/opensuse.md) to give openSUSE admins the opportunity to use Zypper's update lifecycle also for RethinkDB (note that I tested this on openSUSE Tumbleweed with zypper = 1.14.37-1.1. I would be happy if this could be included in RethinkDB's documentation as compiling when there are working binaries at hand is really an unnecessary hassle (and am willing to submit a patch via a pull request).

First, add the CentOS repository to Zypper's repository registry and then update Zypper's cache by running the following commands:

sudo cat << EOF > /etc/zypp/repos.d/rethinkdb.repo
[rethinkdb]
name=RethinkDB
enabled=1
autorefresh=1
baseurl=https://download.rethinkdb.com/repository/centos/8/x86_64/
gpgkey=https://download.rethinkdb.com/repository/raw/pubkey.gpg
gpgcheck=1
path=/
type=rpm-md
keeppackages=0
EOF

sudo zypper refresh

You will be prompted with the following warning message:

Warning: File 'repomd.xml' from repository 'RethinkDB' is unsigned.

    Note: Signing data enables the recipient to verify that no modifications occurred after the data
    were signed. Accepting data with no, wrong or unknown signature can lead to a corrupted system
    and in extreme cases even to a system compromise.

    Note: File 'repomd.xml' is the repositories master index file. It ensures the integrity of the
    whole repo.

    Warning: We can't verify that no one meddled with this file, so it might not be trustworthy
    anymore! You should not continue unless you know it's safe.

File 'repomd.xml' from repository 'RethinkDB' is unsigned, continue? [yes/no] (no):

Accept the unsigned metadata file by entering yes (packages are still cryptographically signed and this warning will not appear again). Next, install RethinkDB:

sudo zypper install rethinkdb

Alternatively, use YaST to add the repository by selecting "Enter URL" when prompted for the repository type and use the URL given above as the base URL. YaST will take care of detecting the correct repository type. Also, you can download the signature key beforehands and add it to Zypper's truststore by clicking on "Keys..." in the YaST repository management interface.

Open 0-getting-started/install/opensuse.md and compare the existing openSUSE installation instructions with the repository setup and installation commands described in the issue. Document the Zypper and YaST paths, including the unsigned metadata warning and key handling, then verify that the page clearly explains how to install RethinkDB from the CentOS repository.