Add CONTRIBUTING.md

Fixes #431

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+# Contributing to Ebiten
+
+Ebiten is an open source project and we would appricate your conributions!
+
+There are some rules for Ebiten contribution.
+
+## Ask us when you are not sure
+
+You can ask us by [creating a GitHub issue](https://github.com/hajimehoshi/ebiten/issues/new) or at [Gophers Slack](https://invite.slack.golangbridge.org/)'s #ebiten channel.
+
+## Follow the Go convention
+
+Please follow the Go convension like [Effective Go](https://golang.org/doc/effective_go.html).
+For example, formatting by `go fmt` is required.
+
+## Add copyright comments to each file
+
+```go
+// Copyright [YYYY] The Ebiten 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.
+```
+
+You don't have to update existing files' license comments.
+
+## Add build tags for examples
+
+```go
+// +build example jsgo
+```
+
+`example` is to prevent from installing executions by `go get github.com/hajimehoshi/ebiten/...`.
+
+`jsgo` is to make the examples work on [jsgo.io](https://jsgo.io/).