Document inline assembly usage according to MISRA-C Directive 4.2

Fixes #181

CMSIS/Documentation/Doxygen/Core/src/misra.md

@@ -4,6 +4,10 @@ CMSIS-Core (Cortex-M) uses the common coding rules for CMSIS components that are
 
 CMSIS-Core (Cortex-M) violates the following MISRA-C:2012 rules:
 
+ - Directive 4.2, All usage of assembly language should be documented
+   - CMSIS-Core uses assembly statements to access core registers on several places. These locations start with __ASM.
+   - Inline assembly statements my be opaque to MISRA Checkers and can cause false-positive warnings.
+
  - Directive 4.9, function-like macro defined.
    - Violated since function-like macros are used to generate more efficient code.