[PATCH] docs/misra: add R18.6 to rules.rst

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

In practice, we are already following R18.6 and we have zero violations
reported by ECLAIR (there are some cautions being reported.)

Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

diff --git a/docs/misra/rules.rst b/docs/misra/rules.rst
index 80e5e972ad..0cb2fb8f24 100644
--- a/docs/misra/rules.rst
+++ b/docs/misra/rules.rst
@@ -580,6 +580,13 @@ maintainers if you want to suggest a change.
      - The relational operators > >= < and <= shall not be applied to objects 
of pointer type except where they point into the same object
      -
 
+   * - `Rule 18.6 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_18_06_1.c>`_
+     - Required
+     - The address of an object with automatic storage shall not be
+       copied to another object that persists after the first object has
+       ceased to exist
+     -
+
    * - `Rule 19.1 
<https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_19_01.c>`_
      - Mandatory
      - An object shall not be assigned or copied to an overlapping