add meta guide

Meta.md

@@ -11,6 +11,9 @@ obj: meta
 - `wiki` - Wiki link
 - `arch-wiki` - Arch Wiki link
 - `rfc` - RFC Document link
+- `rev` - Date of last change (Revision)
+- `mime` - Associated MIME Type
+- `extension` - Associated file extensions
 - `obj` - Object Type
 
 ## Common Frontmatter Tags for Object Types
@@ -26,7 +29,7 @@ obj: meta
 # Object Types
 - `Application` - Any application
 - `OS` - Operating System
-- `Codec` - Media Codec
+- `Format` - File Format
 - `FileSystem` - File System
 - `Emulator` - Emulator System
 - `Concept` - Concept