concept/rules: state that a rule cannot be executed in parallel

Author: dilyanpalauzov

concepts/rules.md

@@ -21,6 +21,9 @@ openHAB can send a notification to your phone.
 Other systems may have a concept of _Automations_, _Tasks_, and other terms.
 In openHAB, rules are used to implement all of these concepts.
 
+The execution of one and the same rule does not happen in parallel.  If a rule is triggered for execution, while the rule is currently run, it will be queued and run later.
+There is no need to program protections against race conditions.
+
 ## Parts of a Rule
 
 These rules take the high level form of _When \_\_t\_\_ happens, if \_\_c\_\_ then do \_\_a\_\__,