add contributing.md

Author: ColdIV

CONTRIBUTING.md

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+# Contributing to Script Manager (SCM)
+Thanks for your interest in SCM. Our goal is to make developing in CompuerCraft as easy as possible.
+
+## Getting Started
+If you want to contribute you can take a look at the existing issues [here](https://github.com/mc-cc-scripts/script-manager/issues).
+
+You can create a pull request to resolve an issue which will then be reviewed by one of the core contributors.
+
+- Make sure to always create a branch from `master` and check if it will merge cleanly before submitting your PR.
+- If your PR is not getting reviewed, feel free to `@` a core contributor in the comments or your PR.
+- Small commits are preferred but should be reasonable. (Don't put every text-change in an extra commit)
+- If possible, add test a test program to your PR. You can just include a pastebin code.