Create README.md

Author: jonasjabari

docs/start/README.md

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+# Matestack UI Core Docs
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+Welcome to the docs of the Ruby gem `matestack-ui-core`, enabling you to easily create component based (reactive) web UIs in pure Ruby on top of your Rails application.
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+The docs cover three main topics of `matestack-ui-core`
+
+- UI components
+- Reactive components
+- Reactive apps