Clarification in index.html

Thought it is pretty obvious to us, index.html should make it clear that the test suite currently only tests XML and SVG implementations of DOM L1 Core.  That when you are running the ECMAScript tests in IE, you are testing the MSXML2 component (or Adobe SVG if you select svg_alltests.html) and when you are running the ECMAScript tests in Mozilla, you are testing their XML parser.  Later revisions will add tests for DOM L1 conformance for HTML implementations.