another run at improving docs

docs/operators/handbook.md

@@ -248,3 +248,13 @@ When referencing an `app` service in a config file, you should prefix with the `
 ## Domain sharing for apps
 
 At time of writing (Jan 2022), we think there is a limitation in our design which doesn't support multiple apps sharing the same domain (e.g. `example.com/app1/` & `example.com/app2/`). This may be possible to overcome if someone really needs it, we encourage people to investigate. We've found that often, there are limitations in the actual software which don't support this anyway and several of the current operators simply use a new domain per app.
+
+## Validating `abra` binary checksums
+
+ You can download `abra` yourself from the [releases page](https://git.coopcloud.tech/coop-cloud/abra/releases) along with the `checksums.txt` file.
+
+```bash
+grep $(sha256sum abra_[version]_[platform]) checksums.txt > /dev/null && echo "checksum OK"
+```
+
+If "checksum OK" appears in your terminal - you're good to go! Otherwise, you have downloaded a corrupted file.