Add CONTRIBUTING.md

Author: kmyk

CONTRIBUTING.md

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+# Contribution and Hacking Guide
+
+links:
+
+-   [CONTRIBUTING.md](https://github.com/online-judge-tools/.github/blob/master/CONTRIBUTING.md) of [online-judge-tools](https://github.com/online-judge-tools) organization
+-   [DESIGN.md](https://github.com/online-judge-tools/verification-helper/blob/master/DESIGN.md)
+
+
+## How to add a new language / 新しい言語を足すには
+
+Do the following steps:
+
+1.  Open an issue about the support for your language
+1.  Make a file `onlinejudge_verify/languages/YOUR_LANGUAGE.py`
+    -   Implement a sub-class of `onlinejudge_verify.languages.models.Language`
+    -   Implement a sub-class of `onlinejudge_verify.languages.models.LanguageEnvironment`
+1.  Register your language at `onlinejudge_verify/languages/list.py`
+1.  Add tests to `tests/` dir if possible
+1.  Update documents `.verify-helper/docs/static/document.md` (or `.verify-helper/docs/static/document.ja.md`)