WIP Mac App Store distribution, CSS enhancements, refactoring filters.mjs

CONTRIBUTING.md

+# Contributing
+
+## What is needed?
+
+### Refactor JavaScript
+sleek is written in plain vanilla JavaScript. I'm doing my best to keep the code as slim and tidy as possible. But I'm not a professional developer and limited on time I can invest in this project. You can contribute by reviewing the JavaScript with the aim of making sleek faster and more maintainable.
+
+### Clean up the CSS
+In order to create sleeks' unique look lot of (S)CSS had been written. If you have experience on CSS you can review and optimise it.
+
+### Work on feature requests
+Over the time plenty of feature requests had been files for sleek: https://github.com/ransome1/sleek/issues. You can simply pick feature request and start working on it. Every feature needs to be contributed with a sufficient amount of test cases (see "Write test cases"). There is a prioritised backlog too: https://github.com/ransome1/sleek/projects/2
+
+### Enhance the CI process
+sleek is being distributed through Github and many other platforms (Snap Store, Flathub, Arch User Repository, Windows Store, Mac App Store). Releasing sleek is still plenty of work and it would be of great help if the releasing process would be further automated. For instance the build process on Flathub or AUR could be triggered on tag creation on Github. The current CI configuration can be found here: https://github.com/ransome1/sleek/blob/develop/.github/workflows/github-ci.yml
+
+### Enhance the design
+If you have interface design experience you can propose design changes. As sleek has a strong focus on its design all changes should be discussed before any implementation is being done.
+
+### Translate sleek into your language
+Plenty of languages are already available, but if yours is missing, you can translate the app to it.
+
+## Write test cases
+
+
+## File pull request
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