Update build instructions (#2136)

BUILDING.md

 See the [release](https://github.com/rancher/k3s/releases/latest) page for pre-built releases.
 
 The clone will be much faster on this repo if you do
-
 ```bash
 git clone --depth 1 https://github.com/rancher/k3s.git
 ```
 
 This repo includes all of Kubernetes history so `--depth 1` will avoid most of that.
 
-To build the full release binary run `make` and that will create `./dist/artifacts/k3s`.
-
-Optionally to build the binaries using local Go environment without running linting or building docker images:
-
+The k3s build process requires some autogenerated code and remote artifacts that are not checked in to version control.
+To prepare these resources for your build environment, run:.
 ```bash
-./scripts/download && ./scripts/build && ./scripts/package-cli
+mkdir -p build/data && ./scripts/download && go generate
 ```
 
-For development, you just need go 1.12+ and a proper GOPATH.  To compile the binaries run:
+To build the full release binary, you may now run `make`, which will create `./dist/artifacts/k3s`.
 
+To build the binaries using without running linting (ie; if you have uncommitted changes):
 ```bash
-go build -o k3s
-go build -o kubectl ./cmd/kubectl
-go build -o hyperkube ./vendor/k8s.io/kubernetes/cmd/hyperkube
+SKIP_VALIDATE=true make
 ```
 
-This will create the main executable at `./dist/artifacts` , but it does not include the dependencies like containerd, CNI,
-etc.  To run a server and agent with all the dependencies for development run the following
-helper scripts:
-
+If you make any changes to go.mod and want to update the vendored modules, you should run the following before runnining `make`:
 ```bash
-# Server
-./scripts/dev-server.sh
-
-# Agent
-./scripts/dev-agent.sh
+go mod vendor && go mod tidy
 ```
 
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