Update documentation to describe the use of the directive [flatpickr]

[andreidiaconescu]

Is your feature request related to a problem? Please describe.
Please Update documentation to describe the use of the directive [flatpickr]

Describe alternatives you've considered
I have looked in the code and found that the directive exists, and used it. I needed a more flexible way to use the date picker, using my html, not the one of the component, so i ended up using the directive.

Thank you.