docs: Introduce CONTRIBUTING guide

Signed-off-by: Alexander Kurbatov <[email protected]>

CONTRIBUTING.md

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+Hi, if you are reading this is because you pretty likely want to participate in the development of this bot and curious about how to start.
+
+## What can I do?
+
+- [ ] Search for other issues tagged [with `good first issue` or `help wanted` tag](https://github.com/alkurbatov/suvorov-bot/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22).
+- [ ] Suggest new features.
+- [ ] Vote for existing issues (issues with highest number of reactions are likely to be fixed first).
+- [ ] Fix `// FIXME: ` comments in the code.
+- [ ] Improve the inline documentation.
+- [ ] Add/Fix/Improve tests.
+- [ ] Just [buy me a coffee](https://www.buymeacoffee.com/alkurbatov).
+
+If you have any suggestions, feel free to comment on this thread, or chat me over [Discord](https://discordapp.com/invite/Emm5Ztz) (the `#cpp` channel).
+
+## What should I know before the start?
+
+**Code guidelines**
+We do our best to conform to [the Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html) with the exception that we use four space tabs instead of two space tabs.
+
+**Commit guidelines**
+We follow the semantic versioning approach, this is important so your PR will appear on the Changelog 🎉 ! I guess you expect recognition and we will give it to you.
+See [standard-version](https://github.com/conventional-changelog/standard-version#commit-message-convention-at-a-glance) for commit guidelines.
+
+The list of supported commit message prefixes:
+
+- `feat` - a new feature or API improvements;
+- `fix` - a bugfix;
+- `perf` - performance improvements;
+- `refactor` - code refactoring;
+- `test` - functional or unit tests;
+- `build` - build system improvements,  also includes CI improvements;
+- `docs` - documentation improvements;
+- `chore` - routine task, e.g. create new release (doesn't appear in the changelog);
+- `style` - fix code style, no functional changes (doesn't appear in the changelog.
+
+## Before PR
+
+* Keep the PR small as possible and one single topic for each.
+* Write good commit message: describe what was the problem and why the changes required.
+* Consider implementing a test.