Common README files for IDP users

**is your feature request related to a problem? Please describe.**
I'd like to add a read-only README.md to the private upload folders of IDP accounts. this config snippet:
```
[/users/${u}/private/README.md]
/srv/files/README.md
accs:
r: *
```
produces this error:
```
ERROR: if ${u} is mentioned in a volume definition, it must be included in both the filesystem-path [/srv/files/README.md] and the volume-url [/users/${u}/private/README.md]
```
which is understandable but unfortunate.

**Describe the idea / solution you'd like**
it makes sense that you can't normally have Quantum Magic Volumes like this, but ideally copyparty should be smart enough to suppress the warning if the volume is read-only and therefore cannot fall victim to concurrent modification shenanigans

**Describe any alternatives you've considered**
I could probably modify my instance's theme, but that sounds like way more work than opening a github issue :3