Updated Liquid for Programmers (markdown)

Liquid-for-Programmers.md

@@ -179,4 +179,10 @@ If you get errors like `A copy of ... has been removed from the module tree but
 ```ruby
 # in config/environments/development.rb and config/environments/test.rb
 Liquid.cache_classes = false
+```
+
+## Difference Between Assigns and Registers
+When you render a template, you can pass it assigns and registers.  The difference is subtle: assigns are exposed to the template, while registers are exposed to Drops, Tags, and Filters.
+```
+@template.render({}, registers: { user: @user })
 ```