chore(*): add few information about BG size

wiki/Backgrounds.md

@@ -11,16 +11,18 @@ A tile’s colors are defined by the tile’s color palette.
 
 At least, a palette is a collection of either 4, 16 or 256 colors.
 
-In order to render background, you has to set few things for each background, remember to set this for every background you want to render :
+In order to render background, you has to set few things for each background, remember to set this for every background you want to render.
 
 ## Background Mode (affects number of planes, bitdepth)
 
-Size of tile (8×8 or 16×16)
-Size of background in tile representation (32×32 to 64×64)
-Set background map
-Set background tileset
-Background modes
 The SNES has 8 background modes.
+- Named Mode 0-7
+- Mode 1 has a variation mode
+- Mode 7 has a submode called Mode 7 Ext. BG
+- Each mode has a different amount of background planes available
+- Each mode has a different amount of colors for the available background planes for that mode
+- Each mode has slightly different rendering properties
+- For all modes and for all graphics planes, color index 0 for any subpalette is the transparency color.
 
 ```
 Mode    # Colors for BG
@@ -39,6 +41,22 @@ Mode    # Colors for BG
 In all modes and for all BGs, color 0 in any palette is considered transparent.
 
 ## Background sizes
+
+In VRAM there is a two-dimensional array that is a map of the tiles on the screen. Depending on the setting of registers $2107 to $210A, this map may be 32x32, 32x64, 64x32 or 64x64 tiles in size.
+
+All tilemaps are 32x32, bits 0-1 simply select the number of 32x32 tilemaps and how they're laid out in memory.
+
+```
+00  32x32   AA
+            AA
+01  64x32   AB
+            AB
+10  32x64   AA
+            BB
+11  64x64   AB
+            CD
+```
+
 ## Background in VRAM
 ## Backgrounds with PVSnesLib
 ## Converting with gfx4snes