Added a contributing guide

.github/CONTRIBUTING.md

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+# Contributing to Jasmine
+
+## Bug reports
+
+Bug reports are welcome, but please help us help you by:
+
+* Searching for existing issues (including closed issues) that are similar to
+  yours
+* Reading the [FAQ](https://jasmine.github.io/pages/faq.html)
+* Providing enough information for someone else to to understand and reproduce
+  the problem. In most cases that includes a clear description of what you're
+  trying to do, the version of each Jasmine package that you're using, the Node
+  version, and a minimal but complete code sample that demonstrates the problem.
+
+## Contributing documentation
+
+We welcome efforts to improve Jasmine's documentation. The source code for the
+documentation site is at <https://github.com/jasmine/jasmine.github.io/>.
+
+## Contributing code
+
+Contributions are welcome, but we don't say yes to every idea. We recommend
+opening an issue to propose your idea before starting work, to reduce the risk
+of getting a "no" at the pull request stage.
+
+Don't have an idea of your own but want to help solve problems for other
+people? That's great! Have a look at the list of
+[issues tagged "help needed"](https://github.com/issues?q=is%3Aopen+is%3Aissue+org%3Ajasmine+sort%3Aupdated-desc).
+
+### The nuts and bolts of preparing a pull request
+
+Jasmine is mature software that's downloaded millions of times a week and
+supported by a tiny group of people in their free time. Anything that breaks
+things for existing users or makes Jasmine harder to maintain is a tough sell.
+
+Before submitting a PR, please check that:
+
+* You aren't introducing any breaking changes
+* `npm test` succeeds: tests pass, there are no eslint or prettier errors,
+   and the exit status is 0
+* Your change is well tested: you're reasonably confident that the tests will
+  fail if somebody breaks your new functionality in the future
+* Your code matches the style of the surrounding code
+
+
+We use Circle CI to test pull requests against a variety of operating systems
+and Node versions. Please check back after submitting your PR and make sure
+that the build succeeded.