Add example file for documentation

Author: egregius313

java/ql/src/Security/CWE/CWE-798/HardcodedJwtKey.java

@@ -0,0 +1,26 @@
+// BAD: Get secret from hardcoded string then sign a JWT token
+Algorithm algorithm = Algorithm.HMAC256("hardcoded_secret");
+JWT.create()
+    .withClaim("username", username)
+    .sign(algorithm);
+}
+
+// BAD: Get secret from hardcoded string then verify a JWT token
+JWTVerifier verifier = JWT.require(Algorithm.HMAC256("hardcoded_secret"))
+    .withIssuer(ISSUER)
+    .build();
+verifier.verify(token);
+
+// GOOD: Get secret from system configuration then sign a token
+String tokenSecret = System.getenv("SECRET_KEY");
+Algorithm algorithm = Algorithm.HMAC256(tokenSecret);
+JWT.create()
+    .withClaim("username", username)
+    .sign(algorithm);
+    }
+
+// GOOD: Get secret from environment variable then verify a JWT token
+JWTVerifier verifier = JWT.require(Algorithm.HMAC256(System.getenv("SECRET_KEY")))
+    .withIssuer(ISSUER)
+    .build();
+verifier.verify(token);