update doc for go mod (#168)

BUILDING.md

@@ -46,18 +46,11 @@ make build-windows
 
 To remove all files not manged by git, run `make clean` (be careful!)
 
-## Adding/updating dependencies
+## Managing dependencies
 
-Requires [dep](https://golang.github.io/dep/).
+[Using Go Modules](https://blog.golang.org/using-go-modules) to manage dependencies.
 
-Add new dependencies directly to `Gopkg.toml` and run `dep ensure`.
-
-To update all dependencies, run `make update-deps`.
-
-After doing any action above, please make sure to run `make fix-deps`,
-which will fix some issue with how dependencies are resolved.
-
-Please check in all changes in the `vendor/` directory.
+To update or add new dependencies, run `go get <package name>`.
 
 ## Cutting a new release