Add Cave engine rule.

Author: MichaelHinrichs

rules.ini

@@ -55,6 +55,7 @@ Buddha = ^Data/Config/User\.(?:cfg|lua)$
 Build = (?:^|/)buildlic\.txt$
 C4_Engine = (?:^|/)Data/C4(?:\.pak)?$
 CatSystem2 = ^cs2confx?\.dll$
+Cave = ^Content\.cave$
 ChoiceScript = (?:^|/)SteamChoiceScript-Info\.plist$
 ChromeEngine[] = (?:^|/)ChromEd\.exe$
 ChromeEngine[] = (?:^|/)ChromeEngine[0-9]\.dll$