add go build tags

go.html.markdown

@@ -30,6 +30,12 @@ Go comes with a good standard library and a sizeable community.
 /* Multi-
  line comment */
 
+ /* A build tag is a line comment starting with // +build
+  and can be execute by go build -tags="foo bar" command.
+  Build tags are placed before the package clause near or at the top of the file
+  followed by a blank line or other line comments. */
+// +build prod, dev, test
+
 // A package clause starts every source file.
 // Main is a special name declaring an executable rather than a library.
 package main