Provide rationale for SC2155 in the case of local as explained by @jgallag88 in delphix/appliance-build#339

SC2155.md

@@ -43,6 +43,16 @@ local foo
 foo=$(mycmd)
 ```
 
+### Rationale
+
+The exit status of the command is overridden by the exit status of the creation the local variable. For example:
+
+```bash
+$ f() { local foo=$(false) || echo foo; }; f
+$ f() { local foo; foo=$(false) || echo foo; }; f
+foo
+```
+
 ### Problematic code in the case of `readonly`:
 
 ```sh