README: Add link to documentation site

mrkn · mrkn · commit 1ce376671692 · 2021-03-22T17:20:04.000+09:00
diff --git a/README.md b/README.md
@@ -2,6 +2,10 @@
 
 UnicodePlot provides the feature to make charts with Unicode characters.
 
+## Documentation
+
+https://red-data-tools.github.io/unicode_plot.rb/
+
 ## Install
 
 ```console
