docs: add contributing guidelines

.github/contributing.md

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+# Contributing to BBB for Nextcloud
+
+:+1::tada: First off, thanks for taking the time to contribute!
+
+:warning: Please note that this project is released with a Contributor Code of Conduct. By
+participating in this project you agree to abide by its terms.
+
+There are plenty of ways to give something back to the open source community.
+Following you find a couple of them for BBB for Nextcloud and no matter which
+you choose, we appreciate every help :clap:.
+
+## :bug: Reporting a bug
+Every application depends on people who are testing it in different environments
+and reporting misbehavior or options to enhance the application further. To help
+developers to fix your issue faster, please [open a new issue] for every bug or
+feature request and follow the guidelines below:
+
+- A issue should be self contained, so that everyone can keep track of changes
+  and discussions.
+- Fill the issue template. It requests important information and omitting it,
+  delays the whole process.
+- Provide as many information as you can. Screenshots, Javascript logs and
+  further information is super helpful.
+- Please be kind. Remember that most developers are working in there free time
+  to make this app better.
+
+## :page_facing_up: Contribute code
+Every code contribution is considered valuable, independent from there size and
+small changes can also be contributed directly via Github:
+
+1. Browse to the desired file via
+   [Github](https://github.com/sualko/cloud_bbb/).
+2. Click on the :pencil2: on the top right corner of the file.
+3. Make your changes.
+4. Add a meaning full commit message, prefixed with either `fix:` or `feat:`
+   (e.g. `fix: room recordings`).
+5. Submit your changes.
+
+If you want to contribute larger changes, we recommend the following approach
+which requires basic git knowledge.
+
+1. Fork the repository.
+2. Install all dependencies with `make dev-setup`.
+3. Create a local branch for your fix or feature (e.g. `fix-recording`).
+4. Test your changes with `yarn build` (you can also use `yarn watch`).
+5. Commit your changes and push your created branch to your fork.
+6. Open a new pull request into our master branch.
+
+We use [Conventional Commits], but the precommit hook should mention any issues
+preventing the merge of your pull request.
+
+## :sunflower: Translate
+We are using [Nextclouds Transifex] setup to translate this application.
+
+## :loudspeaker: Promote
+You like this app and you use a blog, Twitter, Facebook or any other social
+network? It would be great if you could write a small post about BBB for
+Nextcloud.
+
+## :dollar: Sponsoring
+If you like to sponsor a specific feature, bug fix or the project in general you
+can also contact any maintainer of this application to support open source
+development.
+
+[open a new issue]: https://github.com/sualko/cloud_bbb/issues
+[Conventional Commits]: https://www.conventionalcommits.org
+[Nextclouds Transifex]: https://www.transifex.com/nextcloud/nextcloud/