Fix documentation issues

I came across this project and found possible enhancements to the
documentation:

- mention that the release binaries can be used directly
- added the `pre-build` to Cross.toml `build` section, which also works
  for all targets
- fixed the architecture in the arm64 example in README.md

Signed-off-by: Sascha Grunert <[email protected]>

README.md

@@ -58,6 +58,10 @@ default to `docker`.
 cargo install cross --git https://github.com/cross-rs/cross
 ```
 
+It's also possible to directly download the pre-compiled [release
+binaries](https://github.com/cross-rs/cross/releases) or using
+[cargo-binstall](https://github.com/cargo-bins/cargo-binstall).
+
 ## Usage
 
 `cross` has the exact same CLI as [Cargo](https://github.com/rust-lang/cargo)
@@ -169,7 +173,7 @@ RUN ...
 This action will be added to the used image, so it won't be ran/built every time you use `cross`.
 
 ```toml
-[target.x86_64-unknown-linux-gnu]
+[target.aarch64-unknown-linux-gnu]
 pre-build = ["dpkg --add-architecture arm64 && apt-get update && apt-get install --assume-yes libfoo:arm64"]
 ```
 

docs/cross_toml.md

@@ -11,6 +11,7 @@ The `build` key allows you to set global variables, e.g.:
 xargo = true
 build-std = true
 default-target = "x86_64-unknown-linux-gnu"
+pre-build = ["apt-get update"] # can also be the path to a file to run
 ```
 
 # `build.env`