Create CONTRIBUTING.md

CONTRIBUTING.md

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+This contribution guide is in progress, improvements are welcome.
+
+### Code style
+
+Any C++ code PRs should aim to follow the style of existing code in the project. 
+
+Using an autoformatter is heavily recommended, but make sure it's configured properly. 
+
+There's currently preconfigured autoformatter rules for: 
+
+  * CLion (IntelliJ) in .idea/codeStyles/Project.xml
+  
+You can use those to configure your own IDE if it's not already on the list.
+
+#### Linting errors and compiler warnings
+
+Try to avoid any currently enabled warnings and try to reduce the amount of linter errors.
+
+#### Spelling
+
+Make sure you spellcheck your code before commiting it.
+
+#### TODO, FIXME
+
+Check before commiting that you haven't forgotten anything, preferably don't leave these in your commits.
+
+#### Licence headers
+
+You should add your name to the comma-space separated list of contributors if there's a license header.
+
+### License
+
+By contributing you agree to licence your code under the repository's general license (which is currently GPL-v3+).