Add instructions for installing the aarch64 GCC toolchain in README.md.

Change-Id: I50bfd4af971d760efefd51536466659ab9a9ba5a

README.md

diff --git a/README.md b/README.md
index 9f64b1b..df30eee 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,14 @@
 $ rustup target add aarch64-unknown-linux-musl
 ```
 
+To compile to ARM targets, you will also need to have an appropriate
+cross-compiler in your path. For Debian-based systems, this can be installed by
+running the following command:
+
+```sh
+$ sudo apt install gcc-aarch64-linux-gnu
+```
+
 Lastly, run the build script for your desired architecture. This will invoke
 cargo for you as well as create an ext2 image in `out/`.