Added rule for this/base prefixes (#123)

Author: bkoelman

Src/Cheatsheet/Cheatsheet.md

@@ -62,6 +62,7 @@ NOTE: Requires Markdown Extra. See http://michelf.ca/projects/php-markdown/extra
 * Don’t pass `null` as the sender parameter when raising an event (AV1235)  
 * Use generic constraints if applicable (AV1240)  
 * Evaluate the result of a LINQ expression before returning it (AV1250)  
+* Do not use `this` and `base` prefixes unless it is required (AV1251)  
 
 <br/>
 **Maintainability**  

Src/Guidelines/1200_MiscellaneousDesignGuidelines.md

@@ -96,3 +96,7 @@ Consider the following code snippet
 	}
 
 Since LINQ queries use deferred execution, returning `query` will actually return the expression tree representing the above query. Each time the caller evaluates this result using a `foreach` cycle or similar, the entire query is re-executed resulting in new instances of `GoldMember` every time. Consequently, you cannot use the `==` operator to compare multiple `GoldMember` instances. Instead, always explicitly evaluate the result of a LINQ query using `ToList()`, `ToArray()` or similar methods.
+
+### <a name="av1251"></a> Do not use `this` and `base` prefixes unless it is required  (AV1251) ![](images/1.png)
+
+In a class hierarchy, it is not necessary to know at which level a member is declared to use it. Refactoring derived classes is harder if that level is fixed in the code.