add a note for C-lang now missing an impl

Author: gregsdennis

_data/validator-libraries-modern.yml

@@ -11,6 +11,8 @@
       date-draft: [2019-09]
       draft: [7, 6, 4, 3]
       license: "AGPL-3.0-only"
+- name: C
+  notes: All known implementations are now obsolete.
 - name: C++
   implementations:
     - name: f5-json-schema

implementations.md

@@ -40,6 +40,9 @@ Validators
   {% for language in validator-libraries %}
   <li>
     {{language.name}} <a id="validator-{% if language.anchor-name %}{{ language.anchor-name }}{% else %}{{ language.name | downcase }}{% endif %}"></a>
+    {% if language.notes %}
+        {{ implementation.notes }}
+    {% endif %}
     <ul>
     {% for implementation in language.implementations %}
         <li>