Formatter / DCAT / Add instruction how to update SEMIC-EU conversion.

Author: fxprunayre

schemas/iso19139/src/main/plugin/iso19139/formatter/eu-geodcat-ap-semiceu/README.md

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+This formatter is based on XSLT provided by SEMIC-EU.
+See https://github.com/SEMICeu/iso-19139-to-dcat-ap/blob/main/iso-19139-to-dcat-ap.xsl
+
+When updating it, preserve the only addition made here ie. `mode="iso19139-to-dcatap"` added to the `xsl:template` element.
+This is required for the formatter to be used in other XSLT eg. in `iso19115-3` schema.