Add documentation page on "What's BIDS and why adopting it"

[smoia]

Detailed Description

Our documentation lacks a short, BIDS-oriented section in which we give the principles of BIDS and we explain what's the advantage in adopting it.
Writing it would be very useful - and finally close #12!
It doesn't have to be extended, something around half an A4 page could be more than enough.

Possible Implementation

There are many resources we can use to write a couple of paragraphs, starting from BIDS' website, its further information paragraph and its Nature Scientific Data paper.
There is also the first part of this video that could be used.

[smoia]

@RayStick @rmarkello , do you think this could be a good first issue?

[RayStick]

Yes, I think if the person is familiar with BIDS and/or looks into those resources, I think it could work as a good first issue. But see what @rmarkello thinks, as well. :)