adding release notes

doc/release-notes/8578-support-extended-iso-639-languages.md

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+The Controlled Vocabulary Values list for the metadata field Language in the Citation block can be extended to include more ISO 639 languages. This process will merge new values into the existing list adding more than 400 additional languages. For best results please load the new citation.tsv file first as this merge will re-order the list in the display based on the 'Value' and 'identifier' fields.
+Any Controlled Vocabulary Values can be extended by using the tag #controlledVocabularyExt instead of the standard #controlledVocabulary tag. Using this tag will ensure the original alternate values will not be deleted, items with equal unique identifier ids, and/or alternate values, will be merged, and the display order will be regenerated based on the display name ('Value') sorted alphabetically with rows having no identifier at the end of the list (i.e. value:'Not Applicable' id:'').
+
+Example of merged language:
+
+` citation.tsv: Dutch nld dut nl`
+
+` language_ISO_639.tsv: Dutch, Flemish dut nld nl`
+
+` Updated in DB as: Dutch, Flemish nld dut nl`
+
+```
+wget https://raw.githubusercontent.com/IQSS/dataverse/v6.3/scripts/api/data/metadatablocks/language_ISO_639.tsv
+curl http://localhost:8080/api/admin/datasetfield/load -H "Content-type: text/tab-separated-values" -X POST --upload-file language_ISO_639.tsv
+```
+