Some notes on sdmx data messages

After some considerable cajoling I have started a short series of
notes regarding the mysterious sdmx-hd and its relation to dxf2 etc
for data exchange. Despite threatening to blog, I've instead placed
these at because I found
that easier. Please take a look and let me know (i) if it is helpful
(ii) any corrections. Will hopefully continue with parts 2 and 3 this

Once all is a bit more settled we can move these into more official