Removed obsolete diagram. Added CONTRIBUTING.md

Author: KyrylR

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
+
+
+# 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."