docs: document the "verity" NvPCR measurements

I forgot this when I posted 32f405074a,
let's add it now.

docs/TPM2_PCR_MEASUREMENTS.md

@@ -293,3 +293,18 @@ volume name, a ":" separator, the UUID of the LUKS superblock, a ":" separator,
 a brief string identifying the unlock mechanism, a ":" separator, and finally
 the LUKS slot number used. Example string:
 `cryptsetup-keyslot:root:1e023a55-60f9-4b6b-9b80-67438dc5f065:tpm2:1`
+
+## PCR/NvPCR Measurements Made by `systemd-veritysetup` + image dissection logic (Userspace)
+
+### NvPCR `verity` (base+2), Verity root hash + signature info of activated Verity images
+
+The `systemd-veritysetup@.service` service as well as any component using the
+image dissection logic (i.e. `RootImage=` in unit files, or `systemd-nspawn
+--image=`, `systemd-tmpfiles --image=` and similar) will measure information
+about activated Verity images before they are activated.
+
+→ **Measured hash** covers the string `verity:`, followed by the Verity device
+name, followed by `:`, followed by a hexadecimal formatted string indicating
+the root hash of the Verity image, followed by `:`, followed by a comma
+separatec list of PKCS#7 signature key's serial (formatted in hexadecimal), `/`, and
+key issuer (formatted in Base64).