docs: Update secure-boot link in readme

Author: timg236

Readme.md

@@ -184,4 +184,4 @@ Example metadata file contents written to `metadata/SERIAL_NUMBER.json`
 
 <a name="secure-boot"></a>
 ## Secure Boot
-This repository contains the low-level tools and firmware images for enabling secure-boot/verified boot on Compute Module 4 and  Compute Module 5.
+See: [secure-boot](docs/secure-boot.md) reference.