docs: Update security process

Using the GitHub security advisory process is only available to those
with privileged access to the repository. So like Cloud Hypervisor
point to the maintainers.

Signed-off-by: Rob Bradford <[email protected]>

Author: rbradford

MAINTAINERS.md

@@ -0,0 +1,5 @@
+# Maintainers
+
+- Rob Bradford - @rbradford
+- Akira Moroo - @retrage
+

README.md

@@ -100,5 +100,4 @@ sha1sum: 5fc086643dea4b20c59a795a262e0d2400fab15f
 
 ## Security issues
 
-Please use the GitHub security advisories feature for reporting issues:
-https://github.com/cloud-hypervisor/rust-hypervisor-firmware/security/advisories/new
+Please contact the maintainers listed in the MAINTAINERS.md file with security issues.