Add documentation about Has function with more than one template

[nykolaslima]

This need to be documented in README:

//Two skills generated using `valid` template
add("skills",  has(2).of(Skill.class, "valid"));
//Two skills, one generated using `java` template and other using `c++` template
add("skills", has(2).of(Skill.class, "java, "c++"));