update readme

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

README.md

@@ -1,3 +1,51 @@
 # ja-netfilter
 
-### A javaagent lib for network filter
+### A javaagent lib for network filter
+
+## Usage
+
+* 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`
+    * some apps support the `JVM Options file`, you can add as a line of the `JVM Options file`.
+    * **WARNING: DO NOT put some unnecessary whitespace characters!**
+
+* edit your own rule list config file. The `ja-netfilter` will look for it in the following order(find one and stop
+  searching):
+    * passed as args of `-javaagent`.
+      eg: `-javaagent:/absolute/path/to/ja-netfilter.jar=/home/neo/downloads/janf_config.txt`
+    * file path in environment variable: `JANF_CONFIG`
+    * file path in `java` startup property: `janf.config`
+      . `eg: java -Djanf.config="/home/neo/downloads/janf_config.txt"`
+    * some apps support the `JVM Options file`, you can add as a line of the `JVM Options file`
+      . `eg: -Djanf.config="/home/neo/downloads/janf_config.txt"`
+    * file path in the same dir as the `ja-netfilter.jar`
+    * file path in your home directory, named: `.janf_config.txt`. `eg: /home/neo/.janf_config.txt`
+    * file path in the subdirectory named `.config` in your home directory. `eg: /home/neo/.config/janf_config.txt`
+    * file path in the subdirectory named `.local/etc` in your home
+      directory. `eg: /home/neo/.local/ect/janf_config.txt`
+    * file path in the directory named `/usr/local/etc`. `eg: /usr/local/etc/janf_config.txt`
+    * file path in the directory named `/etc`. eg: `/etc/janf_config.txt`
+
+* run your java application and enjoy~
+
+## Config file format
+
+```
+[URL]
+EQUAL,https://someurl
+
+[DNS]
+EQUAL,somedomain
+
+# EQUAL     Use `equals` to compare
+# KEYWORD   Use `contains` to compare
+# PREFIX    Use `startsWith` to compare
+# SUFFIX    Use `endsWith` to compare
+# REGEXP    Use regular expressions to match
+```
+
+## Debug info
+
+* the `ja-netfilter` will **NOT** output debugging information by default
+* add environment variable `JANF_DEBUG=1` and start to enable it