Add attribution and source citations for open source release

- Create ATTRIBUTION.md crediting WoWDev Wiki, TrinityCore, MaNGOS,
  and third-party libraries used
- Add WoWDev Wiki format specification links to file format parsers
  (ADT, BLP, DBC loaders)
- Add protocol documentation reference to opcodes.hpp

include/game/opcodes.hpp

@@ -6,6 +6,8 @@ namespace wowee {
 namespace game {
 
 // World of Warcraft 3.3.5a opcodes
+// Values derived from community reverse-engineering efforts
+// Reference: https://wowdev.wiki/World_Packet
 enum class Opcode : uint16_t {
     // Client to Server
     CMSG_PING = 0x1DC,

include/pipeline/adt_loader.hpp

@@ -147,6 +147,7 @@ struct ADTTerrain {
  * ADT terrain loader
  *
  * Loads WoW 3.3.5a ADT (Azeroth Data Tile) terrain files
+ * Format specification: https://wowdev.wiki/ADT
  */
 class ADTLoader {
 public:

include/pipeline/blp_loader.hpp

@@ -50,6 +50,7 @@ struct BLPImage {
  *
  * Supports BLP0, BLP1, BLP2 formats
  * Handles DXT1/3/5 compression and palette formats
+ * Format specification: https://wowdev.wiki/BLP
  */
 class BLPLoader {
 public:

include/pipeline/dbc_loader.hpp

@@ -14,6 +14,7 @@ namespace pipeline {
  *
  * DBC files store game database tables (spells, items, maps, creatures, etc.)
  * Format: Fixed header + fixed-size records + string block
+ * Format specification: https://wowdev.wiki/DBC
  */
 class DBCFile {
 public: