docs: readme for generated certs

Author: Jorres

tests/data/generate-crts/README.md

@@ -5,8 +5,9 @@
 
 ```
 openssl genrsa -out ca.key 2048
-
 openssl req -x509 -new -nodes -key ca.key -sha256 -days 3650 -out ca.crt -subj "/CN=test-root-ca"
 ```
 
 Then use `generate-test-certs.sh` to generate storage and database certs, which then are used in tests.
+
+`ca.srl` is a file created by the script as well.