Document default behavior of nil and discard_empty for string

[shayonj]

We recently learnt about the default behavior of nil and discard_empty for string in optional fields. Adding the same to README, so its easier to reference. From the TODO, looks like it was intended to be added as well :).

[shayonj]

Update commit message, for more clarity.

[eugeneius]

Thanks for writing this up @shayonj - I agree that discard_empty isn't documented well enough, given that I didn't know about it until I saw this 😄

I'm not sure the README is the right place for this though: the rest of the content there is a high-level introduction to the library, and this is an in-depth explanation of a somewhat niche feature.

Maybe it should be added to the Filtering Input wiki page?

[shayonj]

Its been a while with this PR, I think I will close it for now, thanks for the review @eugeneius, hope you are keeping well 😄