Add new rule for public domain license

Reference: #3997
Signed-off-by: Alok Kumar <alokkumarjipura9973@gmail.com>

Author: alok1304

src/licensedcode/data/rules/public-domain_537.RULE

@@ -0,0 +1,15 @@
+---
+license_expression: public-domain
+is_license_tag: yes
+relevance: 100
+ignorable_urls:
+    - https://github.com/stleary/JSON-java/blob/master/LICENSE
+---
+
+<licenses>
+    <license>
+        <name>Public Domain</name>
+        <url>https://github.com/stleary/JSON-java/blob/master/LICENSE</url>
+        <distribution>repo</distribution>
+    </license>
+</licenses>