Use goimports instead of gofmt in go style guide

doc/development/go_guide/index.md

@@ -70,8 +70,14 @@ projects:
 
 - Avoid global variables, even in packages. By doing so you will introduce side
   effects if the package is included multiple times.
-- Use `go fmt` before committing ([Gofmt](https://golang.org/cmd/gofmt/) is a
-  tool that automatically formats Go source code).
+- Use `goimports` before committing.
+  [goimports](https://godoc.org/golang.org/x/tools/cmd/goimports)
+  is a tool that automatically formats Go source code using
+  [Gofmt](https://golang.org/cmd/gofmt/), in addition to formatting import lines,
+  adding missing ones and removing unreferenced ones.
+
+  Most editors/IDEs will allow you to run commands before/after saving a file, you can set it
+  up to run `goimports` so that it's applied to every file when saving.
 - Place private methods below the first caller method in the source file.
 
 ### Automatic linting