add initial makefile

Signed-off-by: cpanato <ctadeu@gmail.com>

Makefile

@@ -0,0 +1,24 @@
+#
+# Copyright 2022 The Sigstore Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+LDFLAGS ?=
+
+.PHONY: build
+build:
+	CGO_ENABLED=0 go build -trimpath -ldflags "$(LDFLAGS)" .
+
+.PHONY: install
+install:
+	CGO_ENABLED=0 go install -trimpath -ldflags "$(LDFLAGS)" github.com/sigstore/gitsign