finish "user guide: addon" section

README.md

@@ -43,4 +43,25 @@ click "Get mitmproxy-ca-cert.pem" below "Firefox".
 Import it: Settings > Privacy & Security > Certificates > View Certificates... > "Authorities" tab > Import... >
 Choose the downloaded cert > Checkbox "Trust this CA to identify web sites" > OK.
 
-## TODO
+## Archiving web sites
+To get the addon, either clone the git repo:
+```bash
+$ git clone https://git.dc09.ru/mitm-archive/addon
+$ cd addon
+```
+… or just download the script:
+```bash
+$ mkdir addon && cd addon
+$ curl https://git.dc09.ru/mitm-archive/addon/raw/branch/main/addon.py >addon.py
+```
+
+Stop mitmproxy if it's still running (<kbd>q</kbd> and then <kbd>y</kbd> for mitmproxy; <kbd>Ctrl+C</kbd> for mitmweb), then re-launch it with the mitm-archive addon: `mitmproxy -s addon.py` (or mitmweb).
+
+**Each HTTP response** that comes to mitmproxy is archived: metadata is in `./archive.db` SQLite database, headers and body are in `./storage/{id}/headers` and `./storage/{id}/body` respectively.
+
+To adjust these paths, set the environment variables:
+```bash
+$ export SQLITE_DB_PATH=archive.db
+$ export STORAGE=storage
+$ mitmproxy -s addon.py
+```