Document and require yarn for makefile

[finishes vmware-tanzu#15]

Author: squeedee

Diff for: Makefile

@@ -1,5 +1,9 @@
 ADDLICENSE ?= go run -modfile hack/tools/go.mod github.com/google/addlicense
 
+ifeq ($(shell command -v yarn && echo yes),)
+    $(error "Yarn (and node) must be installed")
+endif
+
 .PHONY: serve
 serve:
 	echo "Open docs at: http://localhost:1313"

Diff for: README.md

@@ -5,6 +5,8 @@
 - [Hugo](https://github.com/gohugoio/hugo)
   - macOS: `brew install hugo`
   - Windows: `choco install hugo-extended -confirm`
+- [Node](https://nodejs.org/en/)
+- [Yarn](https://classic.yarnpkg.com/lang/en/docs/install)
 
 ## Serve