README: Add explanation on custom theme paths
The README currently states that feedbackd uses the `default.json` theme, whose location is hardcoded into the application. This doesn't reflect the current state of the code. Explain the ability to add custom themes and give some examples to the user. Also mention the `feedbackd-device-themes` repository explicitly for users looking to modify the current `default.json` theme. Adds explanations about the correct naming of custom theme files and theme file precedence when multiple themes exist.
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