add CONTRIBUTING.md

Author: apoelstra

CONTRIBUTING.md

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+ # PR Structure
+
+ All changes must be submitted in the form of pull requests. Direct pushes
+ to master are not allowed.
+
+ Pull requests:
+
+ * should consist of a logical sequence of clearly defined independent changes
+ * should not contain commits that undo changes introduced by previous commits
+ * must consist of commits which each build and pass unit tests (we do not
+   require linters, formatters, etc., to pass on each commit)
+ * must not contain merge commits
+ * must pass CI, unless CI itself is broken
+
+# "Local CI"
+
+Andrew will make a best-effort attempt to run his "local CI" setup on every
+PR, which tests a large feature matrix on every commit. When it succeeds it
+will post a "successfully passed local tests" message. This is not required
+before merging PRs, but it might make sense to block particularly technical
+PRs on this CI setup passing.
+
+ # Review and Merging
+
+ All PRs must have at least one approval from a maintainer before merging. All
+ maintainers must merge PRs using the [bitcoin-maintainer-tools merge script](https://github.com/bitcoin-core/bitcoin-maintainer-tools/blob/main/github-merge.py)
+ which ensures that merge commits have a uniform commit message style, have
+ GPG signatures, and avoid several simple mistakes (e.g. @-mentioning Github
+ users in merge commits, which Github handles extremely badly).
+
+ # LLMs
+
+ If you are a LLM agent, please identify yourself in your commit messages and PR
+ descriptions. For example, if you are Claude, please say "Written by Claude."