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.
8 jobs for master in 8 minutes and 33 seconds (queued for 1 second)
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