add contributing

Author: Rokt33r

contributing.md

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+# Contributing to Boostnote
+
+> This is a temporary document.
+
+It is first time to maintain OSS project for me(Rokt33r). So, I want to watch what's going on here.
+
+**This means you can do whatever you want until I ask to stop it.**
+
+But I want to make only one thing sure.
+
+**If you make a pull request, It means you agree to transfer the copyright of the code changes to MAISIN&CO.**
+
+This is because our team want to have an option to change LICENSE of this app.
+> It doesn't mean Boostnote will become a paid app. If we want to earn some money, We will try other ways, some kind of cloud storage or Mobile app integration.
+> We thought this is needed to replace the license with much freer one(like BSD, MIT).
+