Update docs to reflect new config API

Author: tom-lord

README.md

@@ -148,15 +148,9 @@ When generating examples, the gem uses 3 configurable values to limit how many e
 
 `Rexexp#examples` makes use of *all* these options; `Rexexp#random_example` only uses `max_repeater_variance`, since the other options are redundant.
 
-To use an alternative value, you can either define a different default value:
+### Defining custom configuration values
 
-```ruby
-RegexpExamples::ResultCountLimiters.max_repeater_variance = 5
-RegexpExamples::ResultCountLimiters.max_group_results = 10
-RegexpExamples::ResultCountLimiters.max_results_limit = 20000
-```
-
-Or, simply pass the configuration option as a parameter:
+To use an alternative value, you can either pass the configuration option as a parameter:
 
 ```ruby
 /a*/.examples(max_repeater_variance: 5)
@@ -169,19 +163,41 @@ Or, simply pass the configuration option as a parameter:
   #=> "A very unlikely result!"
 ```
 
+Or, set an alternative value *within a block*:
+
+```ruby
+RegexpExamples::Config.with_configuration(max_repeater_variance: 5) do
+  # ...
+end
+```
+
+Or, globally set a different default value:
+
+```ruby
+# e.g In a rails project, you may wish to place this in
+# config/initializers/regexp_examples.rb
+RegexpExamples::Config.max_repeater_variance = 5
+RegexpExamples::Config.max_group_results = 10
+RegexpExamples::Config.max_results_limit = 20000
+```
+
 A sensible use case might be, for example, to generate all 1-5 digit strings:
 
 ```ruby
 /\d{1,5}/.examples(max_repeater_variance: 4, max_group_results: 10, max_results_limit: 100000)
   #=> ['0', '1', '2', ..., '99998', '99999']
 ```
 
+### Configuration Notes
+
 Due to code optimisation, `Regexp#random_example` runs pretty fast even on very complex patterns.
 (I.e. It's typically a _lot_ faster than using `/pattern/.examples.sample(1)`.)
 For instance, the following takes no more than ~ 1 second on my machine:
 
 `/.*\w+\d{100}/.random_example(max_repeater_variance: 1000)`
 
+All forms of configuration mentioned above **are thread safe**.
+
 ## Bugs and TODOs
 
 There are no known major bugs with this library. However, there are a few obscure issues that you *may* encounter: