Add comments and fix bugs in Makefile

Author: simeg

Makefile

@@ -2,24 +2,40 @@
 
 GITBOOK = $(shell command -v ./node_modules/.bin/gitbook 2> /dev/null)
 LINTER  = $(shell command -v ./node_modules/.bin/markdownlint 2> /dev/null)
+YARN    = $(shell command -v yarn 2> /dev/null)
 
+
+# This is run on Travis to verify linting is OK and that the guide builds
 ci: lint
 	$(GITBOOK) install && $(GITBOOK) build
 
+
+# Deploy new version of the guide. Will update list of contributors
 deploy:
 	sh ./scripts/publish_gitbook.sh
 
+
+# Install dependencies
 deps:
-	yarn
+ifeq ($(YARN),)
+	$(error "yarn is not available, please install it")
+else
+	$(YARN)
+endif
 
+# Install dependencies and gitbook
 install: deps
 	$(GITBOOK) install
 
+
+# Lint markdown files
 lint:
 	@$(LINTER) . --ignore node_modules && echo 'All good 👌'
 
+
+# Start a server that serves the guide locally
 serve:
-ifndef GITBOOK
+ifeq ($(GITBOOK),)
 	$(error "GitBook is not available, please run 'make install' first")
 else
 	$(GITBOOK) serve