doc: conflicting documentation about Skipping missing properties

Description

Frankly I appreciate your library , but the skipMissingProperties is a bad naming. From my understanding what I read in README :

Sometimes you may want to skip validation of the properties **that do not exist in the validating object.** 

versus source code:

    /**
     * If set to true then validator will skip validation of all properties 
     * that are null or undefined in the validating object.
     */
    skipMissingProperties?: boolean;

are not the same. Properties with null or undefined are not missing at all.

I find it very misleading, would like to know what you think. I guess refactoring should not be that hard.

[NoNameProvided]

The source code doc is correct. Care to open a PR?