add download link

Signed-off-by: pengzhile <pengzhile@gmail.com>

.gitignore

@@ -2,3 +2,4 @@
 .idea/
 .DS_Store
 target/
+publish/

README.md

@@ -4,6 +4,7 @@
 
 ## Usage
 
+* download from the [releases page](https://github.com/pengzhile/ja-netfilter/releases)
 * add `-javaagent:/absolute/path/to/ja-netfilter.jar` argument (**Change to your actual path**)
     * add as an argument of the `java` command.
       eg: `java -javaagent:/absolute/path/to/ja-netfilter.jar -jar executable_jar_file.jar`