docs: update README

Author: black-desk

README.md

@@ -45,28 +45,34 @@ Currently supported target are:
 
 ## Usage
 
-[Install](./docs/install.md) cgtproxy,
-then enable and start the systemd service by:
+1. [Install](./docs/install.md) cgtproxy;
 
-```bash
-# Run the line below if you have old cgtproxy running as systemd service already.
-# systemctl daemon-reload
-systemctl enable --now cgtproxy.service
-```
+2. Enable and start the systemd service by:
 
-You could check the nft rules generated by default (empty) configuration by:
+   ```bash
+   # Run the line below if you have old cgtproxy running as systemd service already.
+   # systemctl daemon-reload
+   systemctl enable --now cgtproxy.service
+   ```
+
+   You could check the nft rules generated by default (empty) configuration by:
 
-```bash
-sudo nft list ruleset
-```
+   ```bash
+   sudo nft list ruleset
+   ```
 
-Write your own configuration file according to the [configuration guide],
-place it on /etc/cgtproxy/config.yaml then restart systemd service by:
+   The nft rule set is generated with [the builtin default configuration].
 
-```bash
-systemctl restart cgtproxy.service
-```
+3. Write your own configuration file according to the [configuration guide],
+   place it at `/etc/cgtproxy/config.yaml`
+
+4. Restart systemd service by:
+
+   ```bash
+   systemctl restart cgtproxy.service
+   ```
 
+[the builtin default configuration]: https://pkg.go.dev/github.com/black-desk/cgtproxy/pkg/cgtproxy/config#pkg-constants
 [configuration guide]: ./docs/configuration.md
 
 ## Tips