docs: add CONTRIBUTING.md with development, testing and release instructions

Author: coderbyheart

CONTRIBUTING.md

@@ -0,0 +1,37 @@
+# Contributing
+
+`@nrfcloud/lambda-helpers` is a published library: it provides helper functions
+for AWS Lambda functions used in nRF Cloud and is published to
+[JSR](https://jsr.io/@nrfcloud/lambda-helpers).
+
+## Development setup
+
+1. Ensure you
+   [have access to the NPM repositories](https://nordicsemi.atlassian.net/wiki/spaces/MFLT/pages/1727136233/Nordic+Engineering+Tools+Setup+go+eng-tools#NPM)
+   and
+   [GitHub push access](https://nordicsemi.atlassian.net/wiki/spaces/MFLT/pages/1727136233/Nordic+Engineering+Tools+Setup+go+eng-tools#nRFCloud-Organization).
+1. Get your environment set up by running `npm ci`
+1. Make your changes locally in a git clone of the repo in your own branch.
+1. As you go, commit along the way so that you get type checking, testing, etc.
+   to run.
+
+## Testing
+
+1. Run `npx tsgo` to type-check the project.
+1. Run `npm test` to run the unit tests.
+
+## Releasing a new version
+
+1. Finally, create a commit that packages up all the changes.
+1. Use [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/)
+   (e.g. `fix:`, `feat:`, etc.) to prefix the title.
+1. Reference any applicable Jira tickets (e.g. `NPE-123`) in the commit message.
+1. Push your branch and create a pull request.
+1. Get the code reviewed.
+1. Once approved and CI passes, rebase or squash away!
+1. [`semantic-release` in the Test&Release workflow](.github/workflows/test-and-release.yaml)
+   takes care of creating a new GitHub release and publishing the package to
+   [JSR](https://jsr.io/@nrfcloud/lambda-helpers).
+
+Now that you have a new version of `@nrfcloud/lambda-helpers` published, you can
+bump the dependency in the projects that consume it.