Document that /Zi is not supported yet (but /Z7 works)

Author: Frerich Raabe

README.asciidoc

@@ -82,6 +82,7 @@ line must be true:
 
 * The +/link+ switch must not be present
 * The +/c+ switch must be present
+* The +/Zi+ switch must not be present (+/Z7+ is okay though)
 * There must be exactly one source file present on the command line.
 
 If all the above requirements are met, clcache forwards the call to the