add basic CONTRIBUTING document

Author: avivace

CONTRIBUTING.md

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+# Contribution guidelines
+
+- Read the [gbdev wide](https://github.com/gbdev/meta/blob/master/CONTRIBUTING.md) contribution guidelines
+- Run an English spell checker
+- Try to split the PR in small, self-contained parts so they can be reviewed and merged faster (and independently)
+- Make sure the code you are adding/changing follows the [GB ASM Style guide](https://gbdev.io/guides/asmstyle.html)
+- When adding new content, give first a read to the existing chapters to get an idea of the style and teaching strategies of the document
+- gb-asm-tutorial is powered by the same preprocessors and renderers we developed for Pan Docs. This gives you additional features over plain Markdown, most notably the "custom containers". To see how to use them, read the ["Special markup" paragraph](https://github.com/gbdev/pandocs/blob/master/CONTRIBUTING.md#special-markup) of the Pan Docs' CONTRIBUTING.md