Add web-visible help for reverse-string for advanced tests

Author: codingthat

exercises/practice/reverse-string/.docs/instructions.append.md

@@ -0,0 +1,14 @@
+# Instructions append
+
+## Handling grapheme clusters
+
+- The [built-in `RegEx` class][regex] can be used to find grapheme clusters.
+- The regular expression pattern `\X` matches grapheme clusters.
+- Backslashes (`\` characters) need to be escaped in regular strings (`"\\X"`), but not in [raw strings][literals] (`r"\X"`).
+- `RegEx` has a [method][search_all] that returns an array of `RegExMatch`es.
+- Each match object has [a method][get_string] that returns the search result as a string.
+
+[regex]: https://docs.godotengine.org/en/stable/classes/class_regex.html
+[literals]: https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_basics.html#literals
+[search_all]: https://docs.godotengine.org/en/stable/classes/class_regex.html#class-regex-method-search-all
+[get_string]: https://docs.godotengine.org/en/stable/classes/class_regexmatch.html#class-regexmatch-method-get-string