Add contributing doc

CONTRIBUTING.md

+# Contributing to k3s #
+
+## Getting Help ##
+
+If you have a question about k3s or have encountered problems using it,
+start by [asking in slack](https://slack.rancher.io/).
+
+## Submitting a Pull Request ##
+
+1. Submit an [issue][issue] describing your proposed change.
+2. We will try to respond to your issue promptly.
+3. Fork this repo, develop and test your code changes. See the project's
+   [README](README.md) for further information about working in this repository.
+4. Submit a pull request against this repo's `master` branch.
+    - Include instructions on how to test your changes.
+5. Your branch may be merged once all configured checks pass, including:
+    - The branch has passed tests in CI.
+    - Two reviews from k3s maintainers
+
+## Committing ##
+
+We prefer squash or rebase commits so that all changes from a branch are
+committed to master as a single commit. All pull requests are squashed when
+merged, but rebasing prior to merge gives you better control over the commit
+message.
+
+### Commit messages ###
+
+Finalized commit messages should be in the following format:
+
+```txt
+Subject
+
+Problem
+
+Solution
+
+Validation
+
+Fixes #[GitHub issue ID]
+```
+
+#### Subject ####
+
+- one line, <= 50 characters
+- describe what is done; not the result
+- use the active voice
+- capitalize first word and proper nouns
+- do not end in a period — this is a title/subject
+- reference the GitHub issue by number
+
+#### Problem ####
+
+Explain the context and why you're making that change.  What is the problem
+you're trying to solve? In some cases there is not a problem and this can be
+thought of as being the motivation for your change.
+
+#### Solution ####
+
+Describe the modifications you've made.
+
+If this PR changes a behavior, it is helpful to describe the difference between
+the old behavior and the new behavior. Provide example CLI output, or changed
+YAML where applicable.
+
+Describe any implementation changes which are particularly complex or
+unintuitive.
+
+List any follow-up work that will need to be done in a future PR and link to any
+relevant Github issues.
+
+#### Validation ####
+
+Describe the testing you've done to validate your change.  Give instructions for
+reviewers to replicate your tests.  Performance-related changes should include
+before- and after- benchmark results.
+
+[issue]: https://github.com/rancher/k3s/issues/new
+[slack]: http://slack.rancher.io/
+
+_This contributing doc adapated from Linkerd2's contributing doc._