Add README.md description of First Responder vocabulary.

Author: msporny

README.md

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+# First Responder Vocabulary
+
+During incidents that involved first responders, it is critical for incident
+commanders to understand the collection of people that are at the incident site,
+as well as their skills and training, in order to deploy the right people, to
+the right places, at the right time.
+
+This specification is useful for describing First Responders using Verifiable
+Credentials in a way that is compatible with NIEMOpen, a community-driven,
+common vocabulary that enables efficient information exchange across diverse
+public and private organizations. Verifiable credentials being issued using this
+technology can be utilized by incident commanders to quickly and accurately
+determine the capabilities of their responders as well as vet, in real time, the
+veracity of their qualifications.
+
+The latest version of the vocabulary is available at:
+
+[https://w3id.org/first-responder/](https://w3id.org/first-responder/)