Update icarLocationIdentifierType.md

Introducing uk.mro.herdid to the scheme as a commonly used identifier for UK dairy herds

well-known/icarLocationIdentifierType.md

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 | uk.cph | | A county parish holding (CPH) is designed to identify and trace the location of livestock in the UK. It is an identification number for your farm or business which pinpoints the location of the land. | 34/123/1234 | It's a 9-digit number: the first two digits identify the county, the next three relate to the parish and the last four digits identify the holding. https://www.gov.uk/guidance/register-land-you-use-to-keep-livestock |
 | au.nlis.pic | | Australian Property Identification Code (PIC) for farms, saleyards, and abattoirs. | QABC1234 | Each State issues its own PICs, and the first character indicates the state. For example, see https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/723633/Procedures-for-assigning-property-identification-codes-and-for-managing-and-accessing-registers.pdf |
+| uk.mro.herdid ||UK milk recording number. Provided by milk recording organisations to identify dairy herds in Great Britain and Northern Ireland. Initially introduced and used by the Milk Marketing Board and now commonly used by the dairy industry. | 011234501 | First two digits indicate a location within the UK, followed by 5 herd digits and finally two digits signifying split herds etc. In some circumstances slashes are included in the number. |