docs: Add contributing rules

CONTRIBUTING.md

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+# Contribution rules
+## Commit Messages
+### Message Structure
+A commit messages consists of three distinct parts separated by a blank line: the title, an optional body and an optional footer. The layout looks like this:
+```
+type: subject
+
+body
+
+footer
+```
+The title consists of the type of the message and subject.
+
+### The Type
+
+The type is contained within the title and can be one of these types:
+
+* feat: a new feature
+* fix: a bug fix
+* docs: changes to documentation
+* style: formatting, missing semi colons, etc; no code change
+* refactor: refactoring production code
+* test: adding tests, refactoring test; no production code change
+* chore: updating build tasks, package manager configs, etc; no production code change
+
+### The Subject
+Subjects should be no greater than 50 characters, should begin with a capital letter and do not end with a period.
+
+Use an imperative tone to describe what a commit does, rather than what it did. For example, use change; not changed or changes.
+
+### The Body
+Not all commits are complex enough to warrant a body, therefore it is optional and only used when a commit requires a bit of explanation and context. Use the body to explain the what and why of a commit, not the how.
+
+When writing a body, the blank line between the title and the body is required and you should limit the length of each line to no more than 72 characters.
+
+### The Footer
+The footer is optional and is used to reference issue tracker IDs.
+
+## Example Commit Message
+```
+feat: Summarize changes in around 50 characters or less
+
+More detailed explanatory text, if necessary. Wrap it to about 72
+characters or so. In some contexts, the first line is treated as the
+subject of the commit and the rest of the text as the body. The
+blank line separating the summary from the body is critical (unless
+you omit the body entirely); various tools like `log`, `shortlog`
+and `rebase` can get confused if you run the two together.
+
+Explain the problem that this commit is solving. Focus on why you
+are making this change as opposed to how (the code explains that).
+Are there side effects or other unintuitive consequenses of this
+change? Here's the place to explain them.
+
+Further paragraphs come after blank lines.
+
+ - Bullet points are okay, too
+
+ - Typically a hyphen or asterisk is used for the bullet, preceded
+   by a single space, with blank lines in between, but conventions
+   vary here
+
+If you use an issue tracker, put references to them at the bottom,
+like this:
+
+Resolves: #123
+See also: #456, #789
+```