make default out substantially more helpful

default_out.md

@@ -1,2 +1,20 @@
+Thanks for installing `rustlings`!
+
+## Is this your first time?
+
+Let's make sure you're up to speed:
+- You have Rust installed, preferably via `rustup`
+- You have `~/.cargo/bin` added to your PATH variable
+- You have cloned this repository (https://github.com/rustlings/rustlings)
+- You have installed Rust language support for your editor
+- You have locally installed the `rustlings` command by running:
+
+```sh
+cargo install --path .
+```
+
+If you've done all of this (or even most of it), congrats! You're ready
+to start working with Rust.
+
 To get started, run `rustlings verify` in order to get the first exercise.
 Make sure to have your editor open!