docs: Add package usage

Signed-off-by: Tim Hårek Andreassen <tim@harek.no>

README.md

@@ -1,6 +1,6 @@
 # [yr]
 
-Get the weather delivered to your command-line.
+Get the weather delivered to your command-line, or use as a Go package.
 
 The project uses [Meteorologisk institutt]'s public API, [`LocationForecast`],
 and [OpenStreetMap]'s [Nominatim] API.
@@ -30,6 +30,20 @@ yr forecast "new york" --web
 yr now "new york" --web
 ```
 
+### As a package
+
+```go
+import "git.sr.ht/~timharek/yr/yr"
+
+func weather() {
+  y := yr.New()
+
+  now := y.Now("bergen")
+
+  // Use the data
+}
+```
+
 ## Contributing
 
 Anyone can contribute to [yr]. Please refer to the [contribution guidelines].