Register format decimal128 (OAI#3209)

* Register format decimal128

* Update decimal128.md

* Apply Darrel's suggestions

registries/_format/decimal128.md

@@ -0,0 +1,31 @@
+---
+owner:
+issue:
+description: A decimal floating-point number with 34 significant decimal digits
+base_type: string, number
+layout: default
+---
+
+# <a href="..">{{ page.collection }}</a>
+
+## {{ page.slug }} - {{ page.description }}
+
+Base type: `{{ page.base_type }}`.
+
+The `{{page.slug}}` format represents a [128-bit decimal floating-point number](https://en.wikipedia.org/wiki/Decimal128_floating-point_format) as defined by IEEE 754 2008 and ISO/IEC/IEEE 60559:2011.
+
+Representation as a JSON string is preferred as this avoids problems with recipients that parse JSON numbers into [binary64](https://en.wikipedia.org/wiki/Double-precision_floating-point_format) memory representation.
+
+String representation allows expressing the special values `NaN`, `-INF`, and `INF` that cannot be expressed as JSON numbers.
+
+{% if page.issue %}
+### GitHub Issue
+
+* [#{{ page.issue }}](https://github.com/OAI/OpenAPI-Specification/issues/{{ page.issue }})
+{% endif %}
+
+{% if page.remarks %}
+### Remarks
+
+{{ page.issue }}
+{% endif %}