add contribution guidelines

[ci skip]

Author: striezel

CONTRIBUTING.md

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+# Contribution guidelines
+
+First off, thanks for taking the time to contribute! :tada::+1:
+
+The following is a set of guidelines for contributing to plotly-node-export-server.
+These are not set in stone, but following these guidelines closely is usually
+considered good practice. Use your best judgment, and feel free to propose
+changes to this document in a pull request.
+
+## Using GitHub to implement features and fix bugs
+
+If you want to help out with one of our bugs or implement a new feature, here
+are some suggestions for success. The basic process looks like this:
+
+1. Fork the repository.
+2. Clone your fork to your development machine.
+3. Create your issue branch.
+4. Implement your feature or fix the bug and push the results to your fork.
+5. Send us a pull request.
+
+You can find more info about [creating a fork](https://help.github.com/articles/fork-a-repo/)
+and [pull request](https://help.github.com/articles/creating-a-pull-request-from-a-fork/)
+in the GitHub help articles. Our maintainers will review your pull request as
+soon as we can.
+
+## What makes a good pull request?
+
+To summarize:
+
+* Address only one issue with a single pull request.
+* For new functionality, have a conversation with the maintainers before putting
+  in too much work. Opening an issue is a good way to start the conversation.
+* Finally, small pull requests are preferred. That way we can find time to
+  review them between work, and family, and watching videos of cute cats jumping
+  into cardboard boxes on the internet.
+
+## Styleguides
+
+### Git Commit Messages
+
+* Use the present tense ("Add feature" not "Added feature").
+* Use the imperative mood ("Move cursor to..." not "Moves cursor to...").
+* Limit all lines of the commit message to 65 characters or less.
+  * Exception: When a line contains a URLs that is too long to fit into the
+    character limit, do not add a line break after 65 characters.
+* URLs (e. g. for linking to information about a vulnerability in a dependency)
+  should be enclose in angle brackets, e. g. `<https://example.com/>`.
+* For multi-line commit messages:
+  * Write the summary on the first line.
+  * Leave the second line empty.
+  * Then follow up with a more detailed explanation of the changes.
+* When only changing documentation, include `[ci skip]` on a separate line in
+  the commit message.
+
+### Code Styleguide
+
+* Try to limit lines to 80 characters.
+* Use spaces (not tabs) for indentation, two spaces per indentation level.