Add brief readme to start documenting the internals of GTE

src/README.md

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+= Rendering Pipeline =
+
+The engine run through the following render loop on every frame
+
+1. Lock in any changes to the play field scroll position
+1. Erase/Redraw dirty sprites into the sprite plane
+   - If a sprite has moved a different amount than the scroll position, it's marked dirty
+   - If a sprite was just added on this frame, it's marked dirty
+   - Any sprite that overlaps a dirty sprite is marked as impacted
+   - All dirty sprites are erased from the sprite plane
+   - All dirty and impacted sprites are drawn into the sprite plane
+   - All of the play field tiles that intersect dirty sprites are marked as dirty with the sprite flag set
+1. If a scroll map is defined
+   - Calculate the new regions of the screen that have been scrolled into view
+   - For each new tile
+      - Copy the tile descriptor from the tile map into the tile store
+      - Mark the tile as dirty
+1. For each dirty tile
+   - Load the tile descriptor from the tile store
+   - Dispatch to the appropriate tile renderer
+   - Clear the tile dirty flag
+1. If any Masked Overlays are defined
+   - Turn off shadowing
+   - Draw the play field on the Overlay rows
+   - Turn on shadowing
+1. In top-to-bottom order
+   - Draw any Maksed Overlays
+   - Draw any Opaque Overlays
+   - Draw any play field rows
+
+*NOTES*
+
+* The dirty tile list has a fast test to see if a tile has already been marked as dirty it is not added twice
+* The tile renderer is where data from the sprite plane is combined with tile data to show the sprites on-screen.
+* Typically, there will not be Overlays defined and the last step of the renderer is just a single render of all playfield lines at once.