Explain "major version number"

https://forum.qubes-os.org/t/7027/

project-security/verifying-signatures.md

@@ -292,8 +292,10 @@ Before we proceed, you must first complete the following prerequisite steps:
 
 The first step is to obtain the correct RSK. The filename of the RSK for your
 Qubes OS release is usually `qubes-release-X-signing-key.asc`, where `X` is the
-major version number of your Qubes release. There are several ways to get the
-RSK for your Qubes release.
+[major version number](https://semver.org/) of your Qubes release. For example,
+if you were installing release `1.2.3`, you would replace `X` with `1`,
+resulting in `qubes-release-1-signing-key.asc`. There are several ways to get
+the RSK for your Qubes release.
 
 - If you have access to an existing Qubes installation, the release keys are
   available in dom0 in `/etc/pki/rpm-gpg/RPM-GPG-KEY-qubes-*`. These can be