New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/28363#issuecomment-770508864

Comment:
Maybe this should have an INSTALL.msg:

From https://raw.githubusercontent.com/matrix-org/synapse/develop/CHANGES.md:

```
This release brings a new schema version for Synapse and rolling back to a previous
version is not trivial. Please review [UPGRADE.rst](UPGRADE.rst) for more details
on these changes and for general upgrade guidance.
```