Add add-tag & push-tag targets to Makefile (open-telemetry#66)

* adding `add-tag` & `push-tag` targets to Makefile

This allows the release process to remain consistent with the collector/contrib repos.

* add newline

Makefile

@@ -63,3 +63,15 @@ goreleaser:
 			exit 1; \
 		fi \
 	}
+
+.PHONY: add-tag
+add-tag:
+	@[ "${TAG}" ] || ( echo ">> env var TAG is not set"; exit 1 )
+	@echo "Adding tag ${TAG}"
+	@git tag -a ${TAG} -s -m "Version ${TAG}"
+
+.PHONY: push-tag
+push-tag:
+	@[ "${TAG}" ] || ( echo ">> env var TAG is not set"; exit 1 )
+	@echo "Pushing tag ${TAG}"
+	@git push upstream ${TAG}