Start with the basics (#675)

* The very first step should be to clone the repository

* Make the command copy-able

By having the plain command users can select and copy the text and paste
it, without needing to fiddle around to remove extra characters.

Now that the previous command switched directories it is not necessary
to be called out here again either.

* Update src/building/how-to-build-and-run.md

Co-authored-by: Yuki Okushi <[email protected]>

Author: badboy

src/building/how-to-build-and-run.md

@@ -6,15 +6,23 @@ be hacking on `rustc`, you'll want to tweak the configuration of the compiler.
 The default configuration is oriented towards running the compiler as a user,
 not a developer.
 
+## Get the source code
+
+The very first step to work on `rustc` is to clone the repository:
+
+```bash
+git clone https://github.com/rust-lang/rust.git
+cd rust
+```
+
 ## Create a config.toml
 
 To start, copy [`config.toml.example`] to `config.toml`:
 
 [`config.toml.example`]: https://github.com/rust-lang/rust/blob/master/config.toml.example
 
 ```bash
-> cd $RUST_CHECKOUT
-> cp config.toml.example config.toml
+cp config.toml.example config.toml
 ```
 
 Then you will want to open up the file and change the following