Add documentation for swagger-parser-safe-url-resolver

modules/swagger-parser-safe-url-resolver/README.md

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+# Swagger Parser Safe URL Resolver
+
+The `swagger-parser-safe-url-resolver` is a library used for verifying that the hostname of URLs does not resolve to a private/restricted IPv4/IPv6 address range.     
+This library can be used in services that deal with user-submitted URLs that get fetched (like in swagger-parser when resolving external URL $refs) to protect against Server-Side Request Forgery and DNS rebinding attacks.
+
+## How does it work?  
+The main class of the package is the `PermittedUrlsChecker` which has one method: `verify(String url)`.  
+This method takes in a string URL and performs the following steps:
+
+1. Gets the hostname portion from the URL  
+2. Resolves the hostname to an IP address
+3. Checks if that IP address is in a private/restricted IP address range (and throws an exception if it is)
+4. Returns a `ResolvedUrl` object which contains   
+  4.1. `String url` where the original URL has the hostname replaced with the IP address  
+  4.2. A `String hostHeader` which contains the hostname from the original URL to be added as a host header  
+
+This behavior can also be customized with the allowlist and denylist in the constructor, whereby:
+
+- An entry in the allowlist will allow the URL to pass even if it resolves to a private/restricted IP address
+- An entry in the denylist will throw an exception even when the URL resolves to a public IP address
+
+## Installation
+Add the following to you `pom.xml` file under `dependencies`
+```xml
+<dependency>
+    <groupId>io.swagger.parser.v3</groupId>
+    <artifactId>swagger-parser-safe-url-resolver</artifactId>
+    // version of swagger-parser being used
+    <version>2.1.14</version> 
+</dependency>
+```
+
+## Example usage
+
+```java
+import io.swagger.v3.parser.urlresolver.PermittedUrlsChecker;
+import io.swagger.v3.parser.urlresolver.exceptions.HostDeniedException;
+import io.swagger.v3.parser.urlresolver.models.ResolvedUrl;
+
+import java.util.List;
+
+public class Main {
+    public static void main() {
+        List<String> allowlist = List.of("mysite.local");
+        List<String> denylist = List.of("*.example.com:443");
+        var checker = new PermittedUrlsChecker(allowlist, denylist);
+
+        try {
+            // Will throw a HostDeniedException as `localhost`  
+            // resolves to local IP and is not in allowlist
+            checker.verify("http://localhost/example");
+
+            // Will return a ResolvedUrl if `github.com` 
+            // resolves to a public IP
+            checker.verify("https://github.com/swagger-api/swagger-parser");
+
+            // Will throw a HostDeniedException as `*.example.com` is 
+            // explicitly deny listed, even if it resolves to public IP
+            checker.verify("https://subdomain.example.com/somepage");
+
+            // Will return a `ResolvedUrl` as `mysite.local` 
+            // is explicitly allowlisted
+            ResolvedUrl resolvedUrl = checker.verify("http://mysite.local/example");
+            System.out.println(resolvedUrl.getUrl()); // "http://127.0.0.1/example"
+            System.out.println(resolvedUrl.getHostHeader()); // "mysite.local"
+        } catch (HostDeniedException e) {
+            e.printStackTrace();
+        }
+    }
+}
+```