Added a Emacs Lisp track-spepcific instruction to "Phone Number".

Author: borderite

exercises/practice/phone-number/.docs/instructions.append.md

@@ -0,0 +1,7 @@
+# Instructions Append
+
+In addition to the function, _numbers_, that cleans up user-entered phone numbers, you need to write two more functions, _area-code_ and _pprint_ in the Emacs Lisp track. Both of the extra functions take the user-entered phone number as their inputs. The first function _area-code_ extracts the area code from the input, while the second function _pprint_ transforms the input to a string in the format of 
+```
+(NXX) NXX-XXXX
+```
+where `N` and `X` are as described above.