docs Add contributors guide (#2)

* feat: added link to list of selectel api urls

* fix: docs in readme

* docs: added CONTRIBUTING.md

* docs: fixed

* fix: stackitcloud -> selectel

---------

Co-authored-by: belokobylskii.i <belokobylskii.i@selectel.com>

CONTRIBUTING.md

@@ -0,0 +1,34 @@
+# Contributing to External DNS Selectel Webhook
+
+Welcome and thank you for making it this far and considering contributing to external-dns-selectel-webhook.
+We always appreciate any contributions by raising issues, improving the documentation, fixing bugs in the CLI or adding new features.
+
+Before opening a PR please read through this document.
+
+## Process of making an addition
+
+To contribute any code to this repository just do the following:
+
+1. Make sure you have a Go version installed that is similar to the version of this project
+2. Fork this repository
+3. Run `make build` to make sure everything's setup correctly
+4. Make your changes
+   > Please follow the [seven rules of greate Git commit messages](https://chris.beams.io/posts/git-commit/#seven-rules)
+   > and make sure to keep your commits clean and atomic.
+   > Your PR won't be squashed before merging so the commits should tell a story.
+   >
+   > Optional: Sign-off on all Git commits by running `git commit -s`.
+   > Take a look at the [Gihub Docs](https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits) for further information.
+   >
+   > Add documentation and tests for your addition if needed.
+5. Run `make lint test` to ensure your code is ready to be merged
+   > If any linting issues occur please fix them.
+   > Using a nolint directive should only be used as a last resort.
+6. Open a PR and make sure the CI pipelines succeed.
+   > Your PR needs to have a semantic title, which can look like: `type(scope) Short description`
+   > All available `scopes` & `types` are defined in [semantic.yml](https://github.com/selectel/external-dns-selectel-webhook/blob/main/.github/semantic.yml)
+   >
+   > An example PR tile for adding a new feature for the CLI would look like: `cli(feat) Add saving output to file`
+7. Wait for two of the maintainers to review your code and react to the comments.
+8. After two approvals merge the PR
+9. Thank you for your contribution! :)