Add CONTRIBUTING.md, simplify PR template (#4619)

This simplifies the PR template, which is a bit cumbersome, and instead
replaces it with a simpler notice that includes a template sign-off and
a new `CONTRIBUTING.md` document.

Signed-off-by: Neil Twigg <neil@nats.io>
Co-authored-by: Byron Ruth <byron@nats.io>

CONTRIBUTING.md

@@ -0,0 +1,45 @@
+# Contributing
+
+Thanks for your interest in contributing! This document contains `nats-io/nats-server` specific contributing details. If you are a first-time contributor, please refer to the general [NATS Contributor Guide](https://nats.io/contributing/) to get a comprehensive overview of contributing to the NATS project.
+
+## Getting started
+
+There are there general ways you can contribute to this repo:
+
+- Proposing an enhancement or new feature
+- Reporting a bug or regression
+- Contributing changes to the source code
+
+For the first two, refer to the [GitHub Issues](https://github.com/nats-io/nats-server/issues/new/choose) which guides you through the available options along with the needed information to collect.
+
+## Contributing changes
+
+_Prior to opening a pull request, it is recommended to open an issue first to ensure the maintainers can review intended changes. Exceptions to this rule include fixing non-functional source such as code comments, documentation or other supporting files._
+
+Proposing source code changes is done through GitHub's standard pull request workflow.
+
+If your branch is a work-in-progress then please start by creating your pull requests as draft, by clicking the down-arrow next to the `Create pull request` button and instead selecting `Create draft pull request`.
+
+This will defer the automatic process of requesting a review from the NATS team and significantly reduces noise until you are ready. Once you are happy, you can click the `Ready for review` button.
+
+### Guidelines
+
+A good pull request includes:
+
+- A high-level description of the changes, including links to any issues that are related by adding comments like `Resolves #NNN` to your description. See [Linking a Pull Request to an Issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) for more information.
+- An up-to-date parent commit. Please make sure you are pulling in the latest `main` branch and rebasing your work on top of it, i.e. `git rebase main`.
+- Unit tests where appropriate. Bug fixes will benefit from the addition of regression tests. New features will not be accepted without suitable test coverage!
+- No more commits than necessary. Sometimes having multiple commits is useful for telling a story or isolating changes from one another, but please squash down any unnecessary commits that may just be for clean-up, comments or small changes.
+- No additional external dependencies that aren't absolutely essential. Please do everything you can to avoid pulling in additional libraries/dependencies into `go.mod` as we will be very critical of these.
+
+### Sign-off
+
+In order to accept a contribution, you will first need to certify that the contribution is your original work and that you license the work to the project under the [Apache-2.0 license](https://github.com/nats-io/nats-server/blob/main/LICENSE).
+
+This is done by using `Signed-off-by` statements, which should appear in **both** your commit messages and your PR description. Please note that we can only accept sign-offs under a legal name. Nicknames and aliases are not permitted.
+
+To perform a sign-off with `git`, use `git commit -s` (or `--signoff`).
+
+## Get help
+
+If you have questions about the contribution process, please start a [GitHub discussion](https://github.com/nats-io/nats-server/discussions), join the [NATS Slack](https://slack.nats.io/), or send your question to the [NATS Google Group](https://groups.google.com/forum/#!forum/natsio).