add instructions on how to download GeoLite2ASN data

Author: phillip-stephens

README.md

@@ -33,3 +33,25 @@ Check that it was installed correctly with:
 ```shell
 zannotate --help
 ```
+
+# Acquiring Datasets
+
+> [!NOTE]
+> URLs and instructions may change over time. These are up-to-date as of September 2025.
+
+Below are instructions for getting datasets from the below providers.
+
+### GeoLite2-ASN
+1. [Sign-up form](https://www.maxmind.com/en/geolite2/signup) for MaxMind GeoLite Access
+2. Login to your account
+3. Go to the "GeoIP / GeoLite" > "Download files" section where you should see a list of available databases
+4. Download the `.mmdb` files for GeoLite ASN
+5. Unzip the downloaded file and test with:
+
+```shell
+echo "1.1.1.1" | zannotate --geoasn --geoasn-database=/path-to-downloaded-file/GeoLite2-ASN_20250923/GeoLite2-ASN.mmdb
+```
+
+```shell
+{"ip":"1.1.1.1","geoasn":{"asn":13335,"org":"CLOUDFLARENET"}}
+```