Add CONTRIBUTING.md (#53)

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CONTRIBUTING.md

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+# Contribute to Collapse OS
+
+Thanks for considering participating to Collapse OS. That the idea of a
+post-collapse operating system sparks your interest is great. However, there
+have been many contributors so far who seem to have misunderstood Collapse OS'
+manifesto. To be clear, let's review its most important points.
+
+## Project goal
+
+Collapse OS' only goal is to preserve humanity's ability to program
+microcontrollers with low-tech tools. We accept only contributions advancing
+the project towards that goal.
+
+Although Collapse OS is mostly a software product, it's also a documentation
+repository. Improving documentation and recipes make the project advance
+towards its goal.
+
+That Collapse OS has such a narrow goal might disappoint many people who are
+enthusiastic about the technological achievement. Sorry for that.
+
+If you think that Collapse OS could be repurposed, please fork it. If it serves
+the general purpose of post-collapse resilience, that project could be part of
+a web of resilience which we'll end up building.
+
+## Priorities and tradeoffs
+
+It is very important for Collapse OS to be self-contained, that is, usable and
+hackable with no external ressources. Because of that, code simplicity is
+paramount.
+
+It is also important for Collapse OS to run on very tight ressources. Because of
+that, code compactness trumps efficiency or features.
+
+All of this means that simplicity is more important than compactness which is
+more important than efficiency.
+
+That being said, common sense still prevails. In a critical loop, we will of
+course favor efficiency.
+
+## Serious
+
+This project is not made with irony. When being faced with existential threat,
+irony offers us a lot of comfort, but this project is not meant to supply this.
+It's self evident that this project is a rich source of possible ironic
+pleasure. To that end, I suggest a fork. Maybe call it "Pip Boy 3000"?
+
+## Roadmap rigidity
+
+This project has a [roadmap](https://collapseos.org/roadmap.html). It says it's
+rigid, but that doesn't mean it's immutable. If you have a good idea that isn't
+on the roadmap, please share it.
+
+## Skill requirements
+
+As the roadmap page says, participation to this project requires a certain set
+of skills, which is true. Authors of this project don't mind helping you out
+honing your skills with code reviews, but please don't expect hand holding.
+
+Important note: the documentation in this project is lacking in many ways. If
+you have problem understanding how Collapse OS work because documentation is
+lacking, please seek "hand holding". This way, we can improve the documentation
+and make post-collapse users much happier.
+
+It is frequently asked by would-be contributors whether I have tips for an
+optimal ramping up of skills. [I've put together a little document to answer
+that question](https://collapseos.org/skills.html).