Add a contribution guide (#455)

* add CONTRIBUTING.md

* Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+# How to contribute
+
+This document describes some points about contribution process for OpenToonz.
+
+## Pull-requests
+
+OpenToonz organization loves any kinds of your contributions, such as fixing typos and code refactroing.
+If you fixed or added something useful to the OpenToonz, please send pull-requests to us.
+We review the request, then we accept it, or add comments for rework, or decline it.
+
+### Workflow
+
+0. `fork` OpenToonz to your GitHub account from `opentoonz/opentoonz`.
+  - (use the `fork` button at the https://github.com/opentoonz/opentoonz)
+0. `clone` the repository.
+  - `git clone git@github.com:your-github-account/opentoonz.git`
+  - `git remote add upstream https://github.com/opentoonz/opentoonz.git`, additionally.
+0. modify the codes.
+  - `git checkout -b your-branch-name`
+    - `your-branch-name` is a name of your modifications, for example,
+      `fix/fatal-bugs`, `feature/new-useful-gui` and so on.
+  - fix codes, then test them.
+  - `git commit` them with good commit messages.
+0. `pull` the latest changes form the `master` branch of the upstream.
+  - `git pull upstream master` or `git pull --rebase upstream master`.
+  - apply [clang-format](http://clang.llvm.org/docs/ClangFormat.html) with `toonz/sources/.clang-format`.
+    - `cd toonz/sources`
+    - `./beautification.sh` or `beautification.bat`.
+  - `git commit` them.
+  - `git push origin your-branch-name`.
+0. make a pull request.
+
+## Bugs
+
+If you found bugs, please report details about them using [issues](https://github.com/opentoonz/opentoonz/issues).
+Then we will try to reproduce the bugs and fix them.
+Unfortunately, sometimes bugs can be only reproduced in your your environment,
+so we cannot reproduce them. We believe you can fix the bug and send us the fix.
+
+## Features
+
+If you had an idea about a new feature, please implement it and send a pull-request to us.
+Even if you cannot implement the feature, you can open a topic in [issues](https://github.com/opentoonz/opentoonz/issues).
+It enables us to discuss about implementaions of the feature there.