instructions for selective generation

Author: fehguy

README.md

@@ -212,6 +212,36 @@ to the code generator like this:
 
 Great for creating libraries on your ci server, from the [Swagger Editor](http://editor.swagger.io)... or while coding on an airplane.
 
+### Selective generation
+You may not want to generate *all* models in your project.  Likewise you may want just one or two apis to be written.  If that's the case, you can use system properties to control the output:
+
+The default is generate *everything* supported by the specific library.  Once you enable a feature, it will restrict the contents generated:
+
+```
+# generate only models
+java -Dmodels {opts}
+
+# generate only apis
+java -Dapis {opts}
+
+# generate only supporting files
+java -DsupportingFiles
+
+# generate models and supporting files
+java -Dmodels -DsupportingFiles
+```
+
+To control the specific files being generated, you can pass a CSV list of what you want:
+```
+# generate the User and Pet models only
+-Dmodels=User,Pet
+
+# generate the User model and the supportingFile `StringUtil.java`:
+-Dmodels=User -DsupportingFiles=StringUtil.java
+```
+
+When using selective generation, _only_ the templates needed for the specific generation will be used.
+
 ### Customizing the generator
 
 There are different aspects of customizing the code generator beyond just creating or modifying templates.  Each language has a supporting configuration file to handle different type mappings, etc: