Create CONTRIBUTING.md (#10)

Author: nakamura-to

CONTRIBUTING.md

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+# Contributing guide
+
+## Legal
+
+All original contributions to Doma are licensed under
+the ASL - [Apache License, version 2.0](https://www.apache.org/licenses/LICENSE-2.0) or later.
+
+## Reporting an issue
+
+This project uses GitHub issues to manage the issues. Open an issue directly in GitHub.
+
+Write the issue in English to share it with many people.
+
+## Before you contribute
+
+To contribute, use GitHub Pull Requests, from your own fork.
+
+## Setup
+
+- Install Git and configure your GitHub access
+- Install JDK 11
+  - We recommend that you use [SDKMAN](https://sdkman.io/jdks) to get JDKs
+- Install GraalVM if you build a native executable
+
+### Build
+
+Clone the repository and navigate to the root directory.
+Then run the Gradle `build` task:
+
+```
+$ git clone https://github.com/domaframework/doma-quarkus.git
+$ cd doma-quarkus
+$ ./mvnw install
+```
+
+### IDE
+
+#### IntelliJ IEDA
+
+Import the root directory as a Maven project.
+
+### Code Style
+
+We use [spotless](https://github.com/diffplug/spotless) and
+[google-java-format](https://github.com/google/google-java-format) 1.7 for code formatting.
+
+To format, just run Maven as follows:
+
+```
+$ ./mvnw spotless:apply
+```
+
+To run google-java-format in your IDE,
+see https://github.com/google/google-java-format#using-the-formatter.
+