Document usage

Author: kddnewton

README.md

@@ -23,6 +23,8 @@ Or install it yourself as:
 
 ## Usage
 
+From code:
+
 ```ruby
 require 'syntax_tree'
 
@@ -34,6 +36,15 @@ raise if parser.error?
 pp node
 ```
 
+From the CLI:
+
+```sh
+$ stree program.rb
+(program
+  (statements
+    ...
+```
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake test` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment.