Add contributing file with lifecycle

CONTRIBUTING.md

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+# Contributing
+
+Thanks for wanting to contribute to Vac specs!
+
+## Issues and PRs
+
+Open an issue with a problem statement and suggested approach, where relevant, and a PR that links to this issue.
+
+If it is a minor cosmetic change this can be pushed as a PR directly.
+
+To make sure people don't miss your PR, ask for review from people of the Vac team in the PR.
+
+## Spec lifecycle
+
+Every spec has its own lifecycle that shows its maturity. We indicate this in a similar fashion to [COSS Lifecycle](https://rfc.unprotocols.org/spec:2/COSS/):
+
+![](assets/lifecycle.png)
+
+## Spec releases and changelog
+
+The way we do releases is:
+
+1. People approve PR
+2. Squash commits in PR
+3. Add one more commit to PR with commit info to previously squashed PR in the "Released on"
+4. Merge rebase
+
+This means the PR results in two commits on master, with the commit hash of a
+release leading to all the changes done in that release.
+
+Changelog is appended to in reverse chronological order with a summary of changes.
+
+In cases where a release hasn't been cut, add a section "Next version" with a
+summary of changes in the PR. This makes it easy to make sure each changelog is
+accurate with actual diff from previous version.
+
+## Questions
+
+Ask these in #vacp2p or #waku, or open an issue.