Improve anagram problem statement

- Add definition of anagram
- Improve readability

exercism/problem-specifications#1528
exercism/problem-specifications#1529

Author: gris

exercises/anagram/README.md

@@ -1,6 +1,7 @@
 # Anagram
 
-Given a word and a list of possible anagrams, select the correct sublist.
+An anagram is a rearrangement of letters to form a new word.
+Given a word and a list of candidates, select the sublist of anagrams of the given word.
 
 Given `"listen"` and a list of candidates like `"enlists" "google"
 "inlets" "banana"` the program should return a list containing