docs: clarify Geo tags purpose and usage in README

Author: joshbuchea

README.md

@@ -174,15 +174,17 @@ These elements provide information for how a document should be perceived, and r
 <meta http-equiv="Window-Target" content="_value">
 
 <!-- Geo tags -->
-<meta name="ICBM" content="latitude, longitude">
-<meta name="geo.position" content="latitude;longitude">
+<meta name="ICBM" content="latitude, longitude"><!-- Geographic coordinates (latitude, longitude) in decimal degrees; eg. content="48.8566, 2.3522" -->
+<meta name="geo.position" content="latitude;longitude"><!-- Geographic coordinates; latitude and longitude are separated by a semicolon -->
 <meta name="geo.region" content="country[-state]"><!-- Country code (ISO 3166-1): mandatory, state code (ISO 3166-2): optional; eg. content="US" / content="US-NY" -->
 <meta name="geo.placename" content="city/town"><!-- eg. content="New York City" -->
 
 <!-- Web Monetization https://webmonetization.org/docs/getting-started -->
 <meta name="monetization" content="$paymentpointer.example">
 ```
 
+**Note:** Geo tags are **not** used by browsers directly — they are intended for search engines, web crawlers, and location-based services to understand the geographic relevance of a page's content. `ICBM` (named after the military ICBM address convention) and `geo.position` both express coordinates in decimal degrees; `ICBM` uses a comma separator while `geo.position` uses a semicolon. `geo.region` identifies the country (and optionally the state/region) using ISO codes, and `geo.placename` provides a human-readable place name.
+
 - 📖 [Meta tags that Google understands](https://support.google.com/webmasters/answer/79812?hl=en)
 - 📖 [WHATWG Wiki: MetaExtensions](https://wiki.whatwg.org/wiki/MetaExtensions)
 - 📖 [ICBM on Wikipedia](https://en.wikipedia.org/wiki/ICBM_address#Modern_use)