add rules

Rules.md

@@ -0,0 +1,28 @@
+# Rules
+
+## How the game plays
+
+The user has at the beginning of the game two words, the first one labelled as the *start word* and the other one as the *end word*.
+
+The goal of the game is to create a path of words between the *start word* and the *end word*. To do this, three types of moves are possible:
+
+1. **Change a single letter**: the user creates a new word using the letters of the previous one, he is allowed to change the order of all letters and additionally to change a single one of them. If a letter appears twice in the word, he can only change one of its occurrences. For example : `mouse --> moose`
+2. **Remove one letter**: the user creates a new word by changing the order of the letters of the previous word and removing one of them. If a letter appears twice in the word, he can only remove one of its occurrences. For example : `muse --> sum`
+3. **Add one letter**: the user creates a new word by changing the order of the letters of the previous word and adding a new letter. For example : `sum --> must`
+
+Each one of the successive words has to be in the dictionary.
+
+Here a complete example:
+
+```
+start word : boy
+end word : girl
+
+boy -> bio # change
+bio -> big # change
+big -> pig # change
+pig -> grip # add
+grip -> girl # add
+```
+
+Once the path is finished its length is computed including the start and end words to compute the number of stars obtained.