add asset system description file

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+# compact asset system for the 64k demo
+
+This file describe the features of a compact asset system used in the demo.
+
+The idea is the following:
+
+# run-time asset retrieval:
+ All assets are const byte arrays. We need a 'const uint8_t* GetAsset(uint16 asset_id)'
+ The asset ID is defined in a 'assets.h' header, generated during the final
+ compile by our own assembling tool. assets.h needs to be re-generated
+ infrequently.
+
+# assembling the assets:
+
+## description file 'assets.txt'
+ All assets are just files in the assets/final/ directory
+ This directory needs a assets.txt text file to describe the asset files.
+ Each line of the assets.txt file contain, comma-separated:
+   * the name of the asset (that will be used by the #define in assets.h),
+   * the name of the file associated
+   * the compression to use for this asset (default NONE. More options later)
+   * and optionally the type of assets.
+
+## example For instance, a line in assets.txt will read:
+
+SAMPLE_142, sample_142.spec, NONE, "this is a drum kick sample"
+
+This instructs the final assembled file assets.h to have a code line:
+  #define ASSET_SAMPLE_142 6323
+
+(6323 is just an associated id)
+
+(or an enum instead of #define's)
+
+so that we can call 
+```
+#include "asset.h"
+const uint8_t* mysample = GetAsset(ASSET_SAMPLE_142);
+...etc
+```
+
+(if we use enums, GetAssert() signature needs to be changed)
+
+### Lazy decompression
+to save memory some assets can be decompressed 'at retrieval time' but kept
+compressed in memory until then.
+This means that we need a 'void DropAsset(uint16 asset_id, const uint8* asset)'
+method to handle memory disallocation depending on the asset type.
+