[#151] updates the changes.txt.

Need clarification on #255 and #254 before we can merge
[#151] Changes text checked

File was slightly amended to deal with clarification over the
document-link element in https://github.com/IATI/IATI-Schemas/issues/255
and https://github.com/IATI/IATI-Schemas/issues/254
Merge pull request #257 from IATI/151-changes-text

151 changes text