[PATCH] Correct src/backend/lib/README

Hi,

I noticed that src/backend/lib/README references binaryheap.c and
stringinfo.c. These files were moved to src/backend/lib. Thus
mentioning them here doesn't look right.

Also README tries to explain the difference between binary heap and
pairing heap. Not sure whether we should keep this text and/or move
the pairing heap implementation closer to binary heap.

-- 
Best regards,
Aleksander Alekseev