Documentation feedback on /docs/install/redhat.rst (SUSE support)

Hi again. Coming from GH-71, we wanted to report that installation on SUSE and derivates fails with an error about signing.

### Problem
```shell
zypper --plus-content=cratedb-ce-stable install -y crate
```

```
Temporarily enabling repository 'CrateDB RPM package repository - x86_64 - Stable'. [--plus-content]
Warning: File 'repomd.xml' from repository 'CrateDB RPM package repository - x86_64 - Stable' 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.
```

```
Repository 'CrateDB RPM package repository - x86_64 - Stable' is invalid.
[cratedb-ce-stable|https://cdn.crate.io/downloads/yum/7/x86_64] Valid metadata not found at specified URL
History:
- Signature verification failed for repomd.xml
```