Update build instructions

Makefile

 BINDIR      := $(CURDIR)/bin
 INSTALL_PATH ?= /usr/local/bin
-DIST_DIRS   := find * -type d -exec
-TARGETS     := darwin/amd64 darwin/arm64 linux/amd64 linux/386 linux/arm linux/arm64 linux/ppc64le linux/s390x windows/amd64
-TARGET_OBJS ?= darwin-amd64.tar.gz darwin-amd64.tar.gz.sha256 darwin-amd64.tar.gz.sha256sum darwin-arm64.tar.gz darwin-arm64.tar.gz.sha256 darwin-arm64.tar.gz.sha256sum linux-amd64.tar.gz linux-amd64.tar.gz.sha256 linux-amd64.tar.gz.sha256sum linux-386.tar.gz linux-386.tar.gz.sha256 linux-386.tar.gz.sha256sum linux-arm.tar.gz linux-arm.tar.gz.sha256 linux-arm.tar.gz.sha256sum linux-arm64.tar.gz linux-arm64.tar.gz.sha256 linux-arm64.tar.gz.sha256sum linux-ppc64le.tar.gz linux-ppc64le.tar.gz.sha256 linux-ppc64le.tar.gz.sha256sum linux-s390x.tar.gz linux-s390x.tar.gz.sha256 linux-s390x.tar.gz.sha256sum windows-amd64.zip windows-amd64.zip.sha256 windows-amd64.zip.sha256sum
 BINNAME     ?= rdpgw
 
 # Rebuild the binary if any of these files change
@@ -37,7 +34,7 @@ all: mod build
 build: $(BINDIR)/$(BINNAME)
 
 $(BINDIR)/$(BINNAME): $(SRC)
-	GOMODULE=on go build $(GOFLAGS) -trimpath -tags '$(TAGS)' -ldflags '$(LDFLAGS)' -o '$(BINDIR)'/$(BINNAME) ./cmd/rdpgw
+	go build $(GOFLAGS) -trimpath -tags '$(TAGS)' -ldflags '$(LDFLAGS)' -o '$(BINDIR)'/$(BINNAME) ./cmd/rdpgw
 
 # ------------------------------------------------------------------------------
 #  install

README.md

@@ -27,10 +27,11 @@ are generated and signed according to the JWT spec by using [jwt-go](https://git
 signed with a 256 bit HMAC. Hosts provided by the user are verified against what was provided by
 the server. Finally, the client's ip address needs to match the one it obtained the token with.
 
-## How to build
+## How to build & install
 ```bash
 cd rdpgw
-go build -o rdpgw .
+make
+make install
 ```
 
 ## Configuration