docs: Add CONTRIBUTING.md and update README with more information

Author: jakubjanecek

CONTRIBUTING.md

@@ -0,0 +1,38 @@
+# Contributing
+
+Thank you for your interest in contributing to Scala Server Toolkit. All contributions are welcome if they are consistent with the goals
+and guidelines of the project.
+
+It is better to open an issue in the project and discuss your intention with project maintainers before you actually start implementing 
+something. They can also give you tips on how to go about implementing it.
+
+## How to Contribute
+
+Please read the [First Contributions Guide](https://github.com/firstcontributions/first-contributions/blob/master/README.md) for general
+information about contribution to OSS projects.
+
+## Build
+
+The project can be built and tested using simple SBT command:
+
+```bash
+sbt test
+```
+
+There is also an extra command called `check` which you should use to check your code before you submit a PR:
+
+```bash
+sbt check
+```
+
+Some of the reported problems can be automatically fixed by `fix`:
+
+```bash
+sbt fix
+```
+
+## Conventional Commits
+
+The project uses [Conventional Commits](https://www.conventionalcommits.org) specification to have clear Git history and help with
+semantic versioning. Please read the specification and follow it (or look at already existing history for inspiration) if you commit into
+the project.

README.md

@@ -26,7 +26,23 @@ the development of your server applications.
 
 ## Issues
 
-Please report issues to [GitHub](https://github.com/avast/scala-server-toolkit/issues).
+Please report issues to [GitHub](https://github.com/avast/scala-server-toolkit/issues). Pull requests are always welcome.
+
+## Versioning
+
+The project follows the [semantic versioning](https://semver.org) schema which means that just by looking at the version you will know 
+whether to expect compatibility problems or not.
+
+The project values backwards compatibility but it cannot prevent innovation. There will be breaking changes but they will always be properly
+versioned and documented.
+
+## Contributing
+
+See [CONTRIBUTING.md](CONTRIBUTING.md).
+
+## Code of Conduct
+
+See [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md).
 
 ## License