Add clarification on differences of is.convert

Closes GH-13.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Co-authored-by: Titus Wormer <tituswormer@gmail.com>

readme.md

@@ -90,6 +90,9 @@ Create a test function from `test`, that can later be called with a `node`,
 Useful if you’re going to test many nodes, for example when creating a utility
 where something else passes an is-compatible test.
 
+The created function is slightly faster because it expects valid input only.
+Therefore, passing invalid input, yields unexpected results.
+
 Can also be accessed with `require('unist-util-is/convert')`.
 
 For example: