Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## What you can do
+
+### Add translations
+
+1. Execute **pylupdate5 $(find -name "*.py") -ts Rare/languages/{your lang (two letters)}.ts** in project directrory
+2. Modify the .ts file manually or in Qt Linguist
+3. Compile the file with **lrelease Rare/languages/{lang}.ts**
+4. Create a Pull request
+
+### Add Stylesheets
+
+For this you can create a .qss file in Rare/Styles/ directory. Here are some exmples: 
+[Qt Docs](https://doc.qt.io/qt-5/stylesheet-examples.html)
+
+### Add features
+
+Select one Feature of the Todo list or improve the code. Then you can create a Pull request