Mention community-maintained themes

Author: st0012

README.rdoc

@@ -1,4 +1,5 @@
 = \RDoc - Ruby Documentation System
+c
 
 home :: https://github.com/ruby/rdoc
 rdoc :: https://ruby.github.io/rdoc
@@ -110,6 +111,33 @@ To determine how well your project is documented run <tt>rdoc -C lib</tt> to
 get a documentation coverage report.  <tt>rdoc -C1 lib</tt> includes parameter
 names in the documentation coverage report.
 
+== Theme Options
+
+There are a few community-maintained themes for \RDoc:
+
+- rorvswild-theme-rdoc[https://github.com/BaseSecrete/rorvswild-theme-rdoc]
+- hanna[https://github.com/jeremyevans/hanna] (a fork maintained by {Jeremy Evans}[https://github.com/jeremyevans])
+
+To use a theme:
+
+- Add the theme to your Gemfile:
+
+    # Use require: false or put it in an appropriate group so it's not loaded by default
+    gem 'theme-gem-name', require: false
+
+- Tell \RDoc to use the theme by
+
+  - Setting the +--format+ option in your +.rdoc_options+ file:
+
+      --format=theme-name
+
+  - Setting the +generator+ option in your RDoc::Task:
+
+      RDoc::Task.new do |rdoc|
+        rdoc.generator = 'theme-name'
+      end
+
+
 == Bugs
 
 See CONTRIBUTING@Bugs for information on filing a bug report.  It's OK to file