Fix guide book to mention rust-toolchain and small build fix

guide/src/guide/getting_started.md

@@ -65,13 +65,16 @@ Before we can write any GPU kernels, we must add a few directives to our `lib.rs
     feature(register_attr),
     register_attr(nvvm_internal)
 )]
+
+use cuda_std::*;
 ```
 
 This does a couple of things:
 - It only applies the attributes if we are compiling the crate for the GPU (target_os = "cuda").
 - It declares the crate to be `no_std` on CUDA targets.
 - It registers a special attribute required by the codegen for things like figuring out
 what functions are GPU kernels.
+- It explicitly includes `kernel` macro and `thread`
 
 If you would like to use `alloc` or things like printing from GPU kernels (which requires alloc) then you need to declare `alloc` too:
 
@@ -191,6 +194,20 @@ static PTX: &str = include_str!("some/path.ptx");
 
 Then execute it using cust.
 
+Don't forget to include the current `rust-toolchain` in the top of your project:
+
+```toml
+# If you see this, run `rustup self update` to get rustup 1.23 or newer.
+
+# NOTE: above comment is for older `rustup` (before TOML support was added),
+# which will treat the first line as the toolchain name, and therefore show it
+# to the user in the error, instead of "error: invalid channel name '[toolchain]'".
+
+[toolchain]
+channel = "nightly-2021-12-04"
+components = ["rust-src", "rustc-dev", "llvm-tools-preview"]
+```
+
 ## Docker
 
 There is also a [Dockerfile](Dockerfile) prepared as a quickstart with all the necessary libraries for base cuda development.