Added brief overview

docs/doc-overview.txt

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+
+lib3ddevil1
+A C library for handling Devil May Cry 1 HD Collection PC file formats.
+These formats are little endian.
+
+Design/Restrictions
+    Library functions do not: 
+        - allocate memory on the heap 
+        - operate on files
+        - convert endianness
+
+Pseudo-Namespaces
+    Library functions are callable through a struct of function pointers.
+    For example:
+        // Print header of a .pld file.
+        DEVIL1PLD.printheader(...);
+
+        // Print header of a texture pack.
+        DEVIL1TEX.printheader(...);
+
+        // Print header of a mesh pack.
+        DEVIL1GEO.printheader(...);
+