Add comments for sample env file

.env.sample

@@ -1,4 +1,9 @@
-PUBLIC_AUTHOR=Sat Naing
+# this is just a sample .env file
+# you can replace any value in the following environment variables
+# note that if you change the default og image name, 
+# you also need to change that image name under public directory
+# also, don't forget to rename this file from ".env.sample" to ".env"
+PUBLIC_AUTHOR=Your Name
 PUBLIC_SITE_DESC=A minimal, responsive and SEO-friendly Astro blog theme.
 PUBLIC_SITE_TITLE=AstroPaper
 PUBLIC_OG_IMAGE=og-default.png