Init API documentation

Author: flekschas

API.md

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+# API
+
+## Data
+
+An array of dictionaries (objects) with two essential properties:
+
+- `id`: the item identifier (has to be unique)
+- `src`: the item data. this can be a URL or data object. the only requirement
+   is that the renderer understands this object.
+
+_Note, mixed data types are currently not supported._
+
+```
+// External image data
+{
+  id: 001,
+  src: 'https://github.com/test.png'
+}
+
+// Matrix data
+{
+  id: 001,
+  src: [[3, 2, 1], [2, 3, 2], [1, 2, 3]]
+}
+```
+