Add Pretend Generations as a rails til.

rails/pretend-generations.md

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+# Pretend Generations
+
+To get an idea of what a `rails generate` command is going to to
+*generate*, you can do a dry run with the `-p` flag or
+the `--pretend` flag. If you run
+
+```
+$ rails generate model post -p
+```
+
+then you will see the following output
+
+```
+    invoke  active_record
+    create    db/migrate/20150513132556_create_posts.rb
+    create    app/models/post.rb
+    invoke    rspec
+    create      spec/models/post_spec.rb
+    invoke      factory_girl
+    create        spec/factories/posts.rb
+```
+
+though those files will not have actually been created. You now know
+precisely what rails will generate for you.