docs: introduce roadmap

In order to establish a foundation to discuss future endeavours we introduce documentation that reflects the current core team decisions

Author: lassediercks

ROADMAP.md

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+# Roadmap
+
+This file provides an overview of the direction solidus is heading.
+
+## [Solidus Admin](https://github.com/orgs/solidusio/projects/12)
+
+- Introduction of hotwire
+- Filter mechanisms for lists. E.g. option types on products, sort orders by state
+- Color themes