Add contribution guide

Former-commit-id: 5d501e16b94c4056efb00ea96e9253aaa00a2578

CONTRIBUTING.md

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+### Contributing code ###
+
+* Writing tests
+* Code review
+* Guidelines
+    - Well-documented. (comments, people, comments!)
+
+
+### Contributing translations ###
+
+* Writing text
+	- You will need to fiddle with .json files in ./res/locales/<Language code>
+	- Do not use double-dash (```--```), use proper em-dash. Em-dashes must not prepend and append any spaces.
+	- Languages with ligatures: please do use them. (e.g. coeur -> cœur)
+	- Japanese: please refrain from using double-hypen (```゠```), use middle dot (```・```)
+* Languagus with apparent grammatical gender
+	- Any gender discrimination should *not* exist in this game, so please choose vocabularies that is gender-neutral. If such behaviour is not possible in the target language, please use male gender, but try your best to avoid the situation.
+
+Note: Right-to-left languages (arabic, hebrew, etc.) are not supported.
+
+
+### Contributing artworks ###
+
+* RGB we mean is always sRGB, _pure white_ we say is always D65 (6 500 K daylight). If you have a monitor calibration device, this is our desired target. (calibration software must be _DisplayCal 3_)
+* Master file for the picture (e.g. psd) can be either RGB/8 or Lab/16.
+* Every exported audio must be in OGG/Vorbis, with quality ```10```.
+* When exporting the image, if the image contains semitransparent colours, export it as TGA; if not, export it as PNG.
+
+        Instruction for exporting the image as TGA
+        
+        1. In Photoshop, File > Export > Quick Export as PNG
+        2. Save the PNG somewhere
+        3. Open up the PNG with Paint.NET/GIMP
+        4. Export the image as TGA
+        5. Include the converted PNG to the project
+
+### Game font ###
+
+The font for the game is managed on [this GitHub repository](https://github.com/minjaesong/Terrarum-sans-bitmap). Make your contribution there and file a pull request on this project.