Document the write command

Author: kddnewton

README.md

@@ -49,6 +49,13 @@ class MyClass
   ...
 ```
 
+or
+
+```sh
+$ stree write program.rb
+program.rb 1ms
+```
+
 ## 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.