Added vocabulary page

ig-source/vocabulary.md

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+# Health Wallet Vocab
+
+## Verifiable Credential (VC) Types
+
+* `https://healthwallet.cards#covid19`: A VC designed to convey COVID-19 details
+* `https://healthwallet.cards#immunization`: A VC designed to convey any immunization details
+* `https://healthwallet.cards#presentation-context-online`: A VC designed for online presentation
+* `https://healthwallet.cards#presentation-context-in-person`: A VC designed for in-person presentation
+
+## FHIR Extensions
+
+ * `https://healthwallet.cards#vc-attachment`: Extension that decorates a FHIR "key resource" to attach a VC
+
+## FHIR Codings
+
+The following codes are defined in the `https://healthwallet.cards` system, for use in tagging a FHIR "key resource" (in `.meta.tag`) as containing a specific type of VC. This facilitates search across FHIR resources to find resources with attached VCs.
+
+
+* `covid19`: Used for tagging a FHIR "key resource" as containing a VC of type `https://healthwallet.cards#covid19`
+
+* `immunization`: Used for tagging a FHIR "key resource" as containing a VC of type `https://healthwallet.cards#immunization`
+
+* `no-did-bound`: Used for `OperationOutcome.issue.code` when the `Patient/:id/$HealthWallet.issueVc` operation fails because no DID is bound to the Patient record