Document pre- and post-condition run methods

Mark pre- and post-condition functionality complete in README.

README.md

@@ -102,7 +102,7 @@ In no particular order, features that have been implemented and are planned:
     - [X] Test multiple and generated values - `sample`, `random_sample`
     - [X] Concise syntax - `given`
     - [X] Before and after hooks - `before_each`, `before_all`, `after_each`, `after_all`, `around_each`
-    - [ ] Pre- and post-conditions - `pre_condition`, `post_condition`
+    - [X] Pre- and post-conditions - `pre_condition`, `post_condition`
     - [ ] Other hooks - `on_success`, `on_failure`, `on_error`
     - [X] One-liner syntax
     - [X] Should syntax - `should`, `should_not`

src/spectator/nested_example_group.cr

@@ -58,11 +58,17 @@ module Spectator
       parent.run_after_each_hooks
     end
 
+    # Runs all of the pre-condition checks.
+    # This method should run prior to every example in the group.
+    # Parent pre-conditions will be checked first.
     def run_pre_conditions : Nil
       parent.run_pre_conditions
       super
     end
 
+    # Runs all of the post-condition checks.
+    # This method should run following every example in the group.
+    # Parent post-conditions will be checked last.
     def run_post_conditions : Nil
       super
       parent.run_post_conditions