[DOC] Add changelog entry

Signed-off-by: Lydia Buntrock <[email protected]>

CHANGELOG.md

@@ -23,3 +23,12 @@ The following API changes should be documented as such:
 
 If possible, provide tooling that performs the changes, e.g. a shell-script.
 -->
+
+## API changes
+
+#### I/O
+
+* To avoid using seqan3::io we changed the usage of the `seqan3::input_file_validator` and
+`seqan3::output_file_validator`, you now have to give the formats explicitly:
+`sharg::input_file_validator validator{std::vector{std::string{"exe"}, std::string{"fasta"}}};` and
+`sharg::output_file_validator validator{sharg::output_file_open_options::create_new, std::vector{std::string{"exe"}, std::string{"fasta"}}};`.