[Retros] Add pieces, first update

[Alain BROBECKER]

Dear retro friends,

Added problem 1C6 (will have to change notation soon
though) and some infos on authors. Problems will be
organised with number of pieces (~ difficulty), and
when there's a tie, the oldest one first. Will have
to add "add N pieces" columns in second table too.

http://abrobecker.free.fr/chess/addpiece.pdf

best, Alain