- adds a base64url format description (#3184)

Author: baywet

registries/_format/base64url.md

@@ -0,0 +1,29 @@
+---
+owner: baywet
+issue:
+description: Binary data encoded as a url-safe string as defined in [RFC4648](https://www.rfc-editor.org/rfc/rfc4648#section-5)
+base_type: string
+layout: default
+deprecated_note: '3.1'
+remarks: "When using OpenAPI 3.1 it's recommended not to use this format and instead use [`contentEncoding` with a value of `base64url`](https://json-schema.org/draft/2020-12/json-schema-validation.html#name-contentencoding)."
+---
+
+# <a href="..">{{ page.collection }}</a>
+
+## {{ page.slug }} - {{ page.description }}
+
+Base type: `{{ page.base_type }}`.
+
+The `{{page.slug}}` format is binary data encoded as a url-safe string as defined in [RFC4648](https://www.rfc-editor.org/rfc/rfc4648#section-5).
+
+{% if page.issue %}
+### GitHub Issue
+
+* [#{{ page.issue }}](https://github.com/OAI/OpenAPI-Specification/issues/{{ page.issue }})
+{% endif %}
+
+{% if page.remarks %}
+### Remarks
+
+{{ page.remarks }}
+{% endif %}