docs: Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# How to contribute
+
+## Submitting changes
+
+Please submit changes by creating a
+[GitHub Pull-Request](https://github.com/simplesurance/bunny-go/pulls).
+
+- A Pull-Request (PR) should contain only one isolated change. Do not mix multiple
+  logical changes in one Pull-Request, e.g. a bugfix, a new feature, an
+  unrelated vendor package upgrade and some formatting changes.
+  Small Pull-Request are easier to review, can be faster processed and merged.
+  They also make it simple to revert specific changes that introduced a bug.
+  Big Pull-Request take longer to get reviewed and merged and risk that a
+  perfect change is rejected because of another unrelated change in the same PR.
+- Add a description about what your Pull-Request changes and why. If you have
+  written descriptive commit messages, they can be reused or referenced as
+  Pull-Request description.
+
+### Commit Messages
+
+- Create a commit per logical change and document it in the commit message.
+  The popular commit message template from
+  [Tim Pope](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html):
+  can be followed:
+
+```
+Capitalized, short (50 chars or less) summary
+
+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 an email 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); tools like rebase can get confused if you run the
+two together.
+
+Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
+or "Fixes bug."  This convention matches up with commit messages generated
+by commands like git merge and git revert.
+
+Further paragraphs come after blank lines.
+
+- Bullet points are okay, too
+
+- Typically a hyphen or asterisk is used for the bullet, followed by a
+  single space, with blank lines in between, but conventions vary here
+
+- Use a hanging indent
+```
+