add a note about windows line endings to readme.md (for #154)

README.md

@@ -131,6 +131,21 @@ maybeUnsafeHTML := markdown.ToHTML(md, nil, nil)
 html := bluemonday.UGCPolicy().SanitizeBytes(maybeUnsafeHTML)
 ```
 
+## Windows / Mac newlines
+
+The library only supports Unix newlines. If you have markdown text with possibly
+Windows / Mac newlines, normalize newlines before caling this library. Use function like:
+
+```go
+func normalizeNewlines(s string) string {
+	// replace CR LF (windows) with LF (unix)
+	s = strings.Replace(s, string([]byte{13, 10}), "\n", -1)
+	// replace CF (mac) with LF (unix)
+	s = strings.Replace(s, string([]byte{13}), "\n", -1)
+	return s
+}
+```
+
 ## mdtohtml command-line tool
 
 https://github.com/gomarkdown/mdtohtml is a command-line markdown to html