Add Postgres validator from PR json-schema-org#141

@pritambaral submitted this info just before the Markdown rewrite
and the conflicts were too substantial for me to resolve in that
PR (as far as I can tell, you can only resolve conflicts within
a file in GitHub, and the whole thing moved to a different file).

So here it is, re-submitted for the current format.

Author: handrews

_data/validator-libraries.yml

@@ -216,4 +216,11 @@
       url: https://github.com/VoxSupplyChain/json-schema-parser
       notes: "Schema parser and validator"
       draft: [4]
-      license: Apache 2.0
+      license: Apache 2.0
+- name: PostgreSQL
+  implementations:
+    - name: postgres-json-schema
+      url: https://github.com/gavinwahl/postgres-json-schema
+      notes: "PL/pgSQL implementation, no remote (http) references"
+      draft: [4]
+      license: PostgreSQL