Added a contributing.md (#1158)

Added a contributing.md

CONTRIBUTING.md

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+# Crucible project status and open source
+
+The Crucible repo is public because it has always been our intention to
+make this open-source.  We thought it was important to explain where we're
+currently at, and manage your expectations.
+
+- We are a small company.
+
+- Our current goal is support our first generation products.
+
+- We're writing Crucible in support of that goal, not as its own thing. We're
+  all working on the products, and block storage is an important product
+  feature.
+
+- Crucible itself has dependencies on many other Oxide repositories, which
+  themselves are undergoing a similar development churn.
+
+- These points together mean that we may not have enough bandwidth to review
+  and integrate outside PRs right now.  We hope this will change in the future.
+
+You're welcome to send PRs, but we want to set expectations right: if we have
+time, or if the PRs are very small or fix bugs, we may integrate them in the
+near future.  But we might also not get to any PR for a while, by which time it
+might no longer be relevant.
+
+We've all dealt with those open source projects that feel open in name only, and
+have big patches and history-free source drops appearing from behind the walls
+of some large organization. We don't like that, and we're not going to do that.
+But it will take some time for us to scale up -- please bear with us.
+
+If you want to ask about whether a PR is consistent with our short-term plan
+_before_ you put in the work -- and you should! -- hit us up on the repo
+Discussions tab on GitHub.
+
+Thanks!