porters-handbook: update Go porting notes for D28184

Approved by:		crees (doc)
Differential Revision:	https://reviews.freebsd.org/D28789

documentation/content/en/books/porters-handbook/special/chapter.adoc

@@ -713,6 +713,31 @@ For ports that use Go, define `USES=go`. Refer to <<uses-go,`go`>> for a list of
 .Creating a Port for a Go Modules Based Application
 [example]
 ====
+In most cases, it is sufficient to set `GO_MODULE` variable to the value specified by `module` directive in `go.mod`:
+
+[.programlisting]
+....
+PORTNAME=       hey
+PORTVERSION=    0.1.4
+DISTVERSIONPREFIX=      v
+CATEGORIES=     benchmarks
+
+MAINTAINER=     dmgk@FreeBSD.org
+COMMENT=        Tiny program that sends some load to a web application
+
+LICENSE=        APACHE20
+LICENSE_FILE=   ${WRKSRC}/LICENSE
+
+USES=           go:modules
+GO_MODULE=      github.com/rakyll/hey
+
+PLIST_FILES=    bin/hey
+
+.include <bsd.port.mk>
+....
+
+If the "easy" way is not adequate or more control over dependencies is needed, the full porting process is described below.
+
 Creating a Go based port is a five stage process. First we need to provide a ports template that fetches the application distribution file:
 
 [.programlisting]

documentation/content/en/books/porters-handbook/uses/chapter.adoc

@@ -519,6 +519,9 @@ Sets default values and targets used to build Go software. A build dependency on
 
 The build process is controlled by several variables:
 
+`GO_MODULE`::
+The name of the application module as specified by `module` directive in `go.mod`. In most cases, this is the only requred variable for ports that use Go modules.
+
 `GO_PKGNAME`::
 The name of the Go package when building in GOPATH mode. This is the directory that will be created in `${GOPATH}/src`. If not set explicitly and `GH_SUBDIR` or `GL_SUBDIR` is present, `GO_PKGNAME` will be inferred from it. It is not needed when building in modules-aware mode.