Adds 'anatomy of planets.py' documentation

README.md

@@ -28,6 +28,9 @@ At startup the current locations of the planets are displayed
 - Press (and hold) **B** to retard the planets
 - Press **X** to exit the application's main loop
 
+As you hold **X** or **B** the speed that the planets advance or retard
+increases, i.e. they orbit faster the longer you press the buttons.
+
 ## Installation
 Copy the files in the `pico` directory to the root of the Raspberry Pi Pico.
 `main.py` will run automatically when the explorer is switched on.
@@ -44,6 +47,21 @@ suitable for the Pico Explorer Base.
 1. `planets.py`: Added `#pylint: skip-file` to line 1
 2. `main.py`: Replaced to use the RV3028 and Pico Explorer Base
 
+**Anatomy of `planets.py`**
+
+The `planets.py` module is unchanged from Dmytro's original. Unfortunately
+it has no documentation, and I've reverse-engineered some potentially
+helpful documentation here...
+
+- `planets_a` is a list of planet 'bitmaps' (spriies). The array has
+  eight entries, one for each planet (Mercury to Neptune). Each array
+  entry is a one-dimensional list of numbers but is 'interpreted' as
+  consecutive blocks (rows) of 5-byte values for each pixel in the bitmap.
+  The five bytes are the pixel's 'x', 'y', 'red', 'green', and 'blue' value.
+  The bitmap size is 7x7 with 'x' and 'y' values ranging from 47 to 50.
+  The pixel at 50,50 (the centre of the bitmap) is placed on planet's
+  _coordinate_.
+
 ---
 
 [dmytro panin]: https://github.com/dr-mod