Issue 134 - Clarify docs on additionalProperties (#135)

* docs additionalProperties=false is a special case 

Clarification in the docs. 
#132

* Spelling

* Readme wording for additional properties updated

Author: romansavchenko

README.md

@@ -227,7 +227,8 @@ console.log(stringify(obj)) // '{"matchfoo":"42","otherfoo":"str","matchnum":3,"
 *additionalProperties* must be an object or a boolean, declared in this way: `{ type: 'type' }`.  
 *additionalProperties* will work only for the properties that are not explicitly listed in the *properties* and *patternProperties* objects.
 
-If *additionalProperties* is not present or is set to `false`, every property that is not explicitly listed in the *properties* and *patternProperties* objects,will be ignored, as described in <a href="#missingFields">Missing fields</a>.  
+If *additionalProperties* is not present or is set to `false`, every property that is not explicitly listed in the *properties* and *patternProperties* objects,will be ignored, as described in <a href="#missingFields">Missing fields</a>.
+Missing fields are ignored to avoid having to rewrite objects before serializing. However, other schema rules would throw in similar situations.
 If *additionalProperties* is set to `true`, it will be used by `JSON.stringify` to stringify the additional properties. If you want to achieve maximum performance, we strongly encourage you to use a fixed schema where possible.  
 Example:
 ```javascript