Add DOCS.md

Custom mixes

DOCS.md

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+### Custom mixes
+
+Mixes can be preloaded and postloaded. Preload mixes will overwrite all content of original mix files and postloaded mix files.
+Postloaded mixes will be overwritten by any other mix files content (include original files).
+
+> Overwite priority it's loading order:\
+Preload mixes -> Original mixes -> Postload mixes
+
+Configuration file (spawn.ini) allow to configure mix loading via two sections: *mixes_preload* for preloading, *mixes_postload* for postloading.
+This section it is just a sorted list of filenames. 
+
+Example:
+
+> [mixes_preload] \
+1=HTNK.mix \
+0=HTK.mix
+
+The first will be HTK.mix and then HTNK.mix - this list will sorted in ascending order.