docs: Clarify you must build the site to have the file generated (#489)

doublejosh · mdreizin · web-flow · commit 4dcf3dcbcbb7 · 2021-10-19T22:38:15.000Z
Co-authored-by: Marat Dreizin &lt;mdreizin@users.noreply.github.com&gt;
diff --git a/README.md b/README.md
@@ -7,7 +7,7 @@
 
 # gatsby-plugin-robots-txt
 
-> Create `robots.txt` for your Gatsby site.
+> Create `robots.txt` for your Gatsby site on build.
 
 ## Install
 
