Add problematic/correct examples for readonly declarations

SC2155.md

@@ -42,3 +42,16 @@ local foo="$(mycmd)"
 local foo
 foo=$(mycmd)
 ```
+
+### Problematic code in the case of `readonly`:
+
+```sh
+readonly foo="$(mycmd)"
+```
+
+### Correct code:
+
+```sh
+foo="$(mycmd)"
+readonly foo
+```