Add Java README

Custom-Headers/Java/README.md

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+# Add Custom Headers
+
+## Instructions
+1. **`git clone https://github.com/UthmanPortSwigger/add-custom-headers.git`**
+2. **`cd add-custom-headers/Custom-Headers/Java`**
+3. Edit **BurpExtender.java** using vim or a text editor of your choice. Change **urlMatchList** to match your scope, **headersToAdd** to match the headers you want to add, and set **checkForDuplicates** to true or false. In the example under Releases (v1.0), the scope is https://portswigger-labs.net and the headers added are "Header1: value1" and "Header2: Value2"
+4. Run **`./gradlew fatJar`**
+5. Load the **Add-Headers-Java.jar** file created at **build/libs** into Burp Suite Professional under **`Extender > Extensions > Add`** or Burp Suite Enterprise under **`Cog/Settings icon > Extensions > Custom extensions > Upload extension`**
+    - https://portswigger.net/burp/documentation/enterprise/working/scans/extensions#writing-and-uploading-your-own-extensions
+
+## Note for Enterprise users:
+- You will need to be running [Burp Suite Enterprise 2021.8](https://portswigger.net/burp/releases/enterprise-edition-2021-8?requestededition=enterprise) or later. Please build this extension with the Java 11 JRE in your `Enterprise installation directory > jres`.<br />
+- The build command in step 4 will then become: `./gradlew -Dorg.gradle.java.home=<path-to-your-enterprise-installation-directory>/jres/11.x/Contents/Home fatJar` or `gradle -Dorg.gradle.java.home=<path-to-your-enterprise-installation-directory>/jres/11.x/Contents/Home fatJar`
+- In the points above, replace `<path-to-your-enterprise-installation-directory>` and `11.x` as appropriate
+
+**General Note:** If you see a `permission denied: ./gradlew` error, please run `chmod +x gradlew` before the build command