docs: started a small contribution guide

docs/contribution_guide.md

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+# Contribution guide
+
+We really appreciate contribution and would love to see you joining the project :)
+But please read this document carefully before starting contribution.
+We probably won't accept most PRs that don't align with the following guidelines.
+
+## Issue before PR
+
+Please only implement features that have an open issue or open an issue before starting to work on a feature.
+By that you don't waste time on a feature, that maybe won't be accepted and we can understand your code more easily.
+
+## Use rustfmt
+
+Please format your code with rustfmt (`cargo fmt`) before committing it.
+By that there won't be any additional commits needed just to align the code with the rust style conventions.
+
+## Update docs
+
+Please keep the docs up to date, when you change anything relevant to the documentation (API, etc.).
+
+## 1 feature 1 commit
+
+Keeping commits limited to one feature makes it way easier to understand them and increases the probability that the
+commit will be accepted.
+
+## commit message convention
+
+Please keep your commit messages in the following style:
+
+```
+type(scope): change
+
+notes
+```
+
+A commits type can be one of the following:
+
+- **feat**: feature
+- **fix**: bugfix
+- **refactor**: nothing new added but code restructured, etc
+- **docs**: documentation changes
+
+The commit scope is kinda free, but it's a good idea to use scopes, that have been used before.
+Scopes can be nested like this: `scope >> subscope >> subscope`.
+If the commit affects multiple scopes, you can set multiple scopes like this: `scope // scope`.