Added metadata documentation.

metadata.md

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+# Metadata definitions
+## Release image
+- A single release of a game. For example: Star Fox 64 (U) v1.2 [!]
+
+    * UUID
+    * sha1sum
+    * filename
+    * release group
+
+
+## Release group
+- A container for all versions of a release.
+  For example, the Star Fox 64 release group would contain:
+    Star Fox 64 (U) v1.2
+    Star Fox 64 (U) v1.0
+    Star Fox 64 (J) v1.0
+    etc.
+
+    * UUID
+    * name
+    * platform
+
+## Platform
+- A single set of hardware and/or software that shares compatibility
+  Examples:
+    Nintendo Entertainment System
+    Microsoft Windows 7
+    Microsoft Windows 98
+
+    * UUID
+    * name
+
+## DAT credits
+- A large list of hashes imported from other sources.
+    * DAT name
+    * DAT website
+    * DAT version
+    * image UUID list