add mod file (#30)

- [go modules](https://github.com/golang/go/wiki/Modules) will be enabled in 1.13.
- after merge it would be nice if you would tag this repo: e.g. `v1.0.0`
  - this will allow consumers something more consistent and readable than `v0.0.0-20181229203832-0af3f3b09a0a`.

go.mod

@@ -0,0 +1,16 @@
+// Copyright 2014 Google Inc.
+//
+// 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.
+module github.com/google/btree
+
+go 1.12