Added usage information to the README file

README.md

@@ -12,3 +12,54 @@ composer require "lindelius/php-fide=^0.1"
 ```
 
 ## Usage
+
+First of all, implement the `Lindelius\FIDE\Contestant` interface in your ladder participant model (the object holding rating information about a given contestant in a given ladder or tournament)
+
+```php
+use Lindelius\FIDE\Contestant;
+
+/**
+ * Class LadderParticipant
+ */
+class LadderParticipant implements Contestant
+{
+    public function currentRating()
+    {
+        // TODO: Return the contestants current rating
+    }
+
+    public function highestRating()
+    {
+        // TODO: Return the contestants highest rating
+    }
+
+    public function totalMatchesPlayed()
+    {
+        // TODO: Return the total number of matches played by the contestant
+    }
+}
+```
+
+then use the static `Lindelius\FIDE\RatingSystem::calculateRatingChange()` method to calculate the contestants rating changes after each match.
+
+```php
+use Lindelius\FIDE\RatingSystem;
+
+/**
+ * Calculate rating changes.
+ * 
+ * `$outcome` should be the outcome for `$contestant` and must be either of the
+ * constants defined in the `Lindelius\FIDE\RatingSystem` class.
+ */
+$contestantRatingChange = RatingSystem::calculateRatingChange($contestant, $opponent, $outcome);
+$opponentRatingChange   = RatingSystem::calculateRatingChange($opponent, $contestant, -$outcome);
+
+/**
+ * Set the contestants new ratings by adding the calculated rating changes to 
+ * their current ratings.
+ */
+$contestantNewRating = $contestant->currentRating() + $contestantRatingChange;
+$opponentNewRating   = $opponent->currentRating() + $opponentRatingChange;
+
+// TODO: Save the new ratings
+```