Updated README with RPM build instructions

This patch updates the README with instructions on building the RPM
locally.
diff --git a/Makefile b/Makefile
index 68c3341..0ba81cb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,15 +1,21 @@
 all: build
 
-build:
-	@mkdir -p rpmbuild/RPMS rpmbuild/SPECS rpmbuild/SRPMS \
-	  rpmbuild/BUILD/etc/cloud/cloud.cfg.d \
-	  rpmbuild/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources
-	docker build -t rpmbuild .
+rpm-centos7:
+	@mkdir -p rpmbuild/centos7/RPMS rpmbuild/centos7/SPECS \
+	  rpmbuild/centos7/SRPMS \
+	  rpmbuild/centos7/BUILD/etc/cloud/cloud.cfg.d \
+	  rpmbuild/centos7/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources
+	docker build -f Dockerfile.rpmbuild -t rpmbuild:centos7 .
 	docker run --rm -it \
 		-v $$(pwd)/rpmmacros:/root/.rpmmacros:ro \
-		-v $$(pwd)/rpmbuild/:/root/rpmbuild \
-		-v $$(pwd)/rpm.spec:/root/rpmbuild/SPECS/rpm.spec:ro \
+		-v $$(pwd)/rpmbuild/centos7:/root/rpmbuild \
+		-v $$(pwd)/rpm.centos7.spec:/root/rpmbuild/SPECS/rpm.spec:ro \
 		-v $$(pwd)/99_vmx_guestinfo.cfg:/root/rpmbuild/BUILD/etc/cloud/cloud.cfg.d/99_vmx_guestinfo.cfg:ro \
 		-v $$(pwd)/DataSourceVmxGuestinfo.py:/root/rpmbuild/BUILD/usr/lib/python2.7/site-packages/cloudinit/sources/DataSourceVmxGuestinfo.py:ro \
-		rpmbuild \
+		rpmbuild:centos7 \
 		rpmbuild -ba /root/rpmbuild/SPECS/rpm.spec
+
+rpm: rpm-centos7
+
+build: rpm
+