Add info about markers in tests in workflow doc

[danilkuznetsov]

Add info about markers in section TDD //given //when //then

about when and how right use this markers.

right example

//given Iterator i = mock(Iterator.class);
given(i.next()).thenReturn("Hello").thenReturn("World");
//when
String result = i.next()+" "+i.next();
//then
assertEquals("Hello World", result);