Document new hadd options

io/doc/v606/index.md

+## I/O Libraries
+
+### hadd
+
+We extended the `hadd` options to allow more control on the compression settings use for the
+output file.  In particular the new option -fk allows for a copy of the input
+files with no decompressions/recompression of the TTree baskets even if they
+do not match the requested compression setting.
+
+New options:
+
+- `-ff` allows to force the compression setting to match the one from the first input
+- `-fk[0-209]` allows to keep all the basket compressed as is and to compress the meta data with the given compression setting or the compression setting of the first input file.
+- `-a` option append to existing file
+- The verbosity level is now optional after -v
+
+### I/O New functionalities
+
+### I/O Behavior change.