(maint) - Add README section on overwrite_fact

README.md

@@ -443,6 +443,22 @@ To do this, pass a lambda as the value for the custom fact. The lambda is passed
 add_custom_fact :root_home, lambda { |os,facts| "/tmp/#{facts['hostname']}" }
 ```
 
+#### Overwrite existing facts
+
+You can also supply an optional input `:overwrite_fact` to the `add_custom_fact` method.
+
+This allows you to completely overwrite a fact already present in the facts hash as oppose to deep merging the fact values.
+
+```ruby
+add_custom_fact :networking, { "ip" => "127.0.0.1", "ip6" => "FE80:0000:0000:0000:AAAA:AAAA:AAAA", "mac" => "AA:AA:AA:AA:AA:AA"}, :overwrite_fact => true
+```
+
+will return the following hash when queried for networking facts
+
+```ruby
+:networking => { "ip" => "127.0.0.1", "ip6" => "FE80:0000:0000:0000:AAAA:AAAA:AAAA", "mac" => "AA:AA:AA:AA:AA:AA"}
+```
+
 ### Supplying Custom External Facts through FacterDB
 Rspec-puppet-facts uses a gem called facterdb that contains many fact sets of various combinations that are pre generated.  Rspec-puppet-facts queries
 facterdb to pull out a specific fact set to use when testing.