Add comment about IE behaviour

Encourage not regressing on this since we don't have automated browser tests yet

Author: ashb

lib/markdown.js

@@ -164,6 +164,7 @@ function count_lines( str ) {
 Markdown.prototype.split_blocks = function splitBlocks( input, startLine ) {
   input = input.replace(/(\r\n|\n|\r)/g, "\n");
   // [\s\S] matches _anything_ (newline or space)
+  // [^] is equivalent but doesn't work in IEs.
   var re = /([\s\S]+?)($|\n#|\n(?:\s*\n|$)+)/g,
       blocks = [],
       m;