doc/Makefile: add 'all' rule to build all docs

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5334044

doc/Makefile

diff --git a/doc/Makefile b/doc/Makefile
index 7957a0d..4d1f4ee 100644
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -8,6 +8,8 @@
 GOFILES=\
 	tmpltohtml.go\
 
+all: tmpltohtml go_tutorial.html effective_go.html
+
 go_tutorial.html: go_tutorial.tmpl tmpltohtml
 	./makehtml go_tutorial.tmpl