docs: link from makefile to compile instructions

Some practitioners look to the makefile directly rather than to the consul
website for information on how to compile from source. Link to the website
instructions directly from the makefile so the practitioner can accomplish
their task successfully without a careful read of the makefile.

GNUmakefile

@@ -1,3 +1,6 @@
+# For documentation on building consul from source, refer to:
+# https://www.consul.io/docs/install#compiling-from-source
+
 SHELL = bash
 GOGOVERSION?=$(shell grep github.com/gogo/protobuf go.mod | awk '{print $$2}')
 GOTOOLS = \