Add some help text.

.cargo/config

@@ -1,13 +1,42 @@
+#
+# Cargo Configuration for the https://github.com/rp-rs/rp-hal.git repository.
+#
+# Copyright (c) The RP-RS Developers, 2021
+#
+# You might want to make a similar file in your own repository if you are
+# writing programs for Raspberry Silicon microcontrollers.
+#
+# This file is MIT or Apache-2.0 as per the repository README.md file
+#
+
 [build]
-# Instruction set of Cortex-M0+
+# Set the default target to match the Cortex-M0+ in the RP2040
 target = "thumbv6m-none-eabi"
 
-[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+# Target specific options
+[target.thumbv6m-none-eabi]
+# Pass some extra options to rustc, some of which get passed on to the linker.
+#
+# * linker argument --nmagic turns off page alignment of sections (which saves
+#   flash space)
+# * linker argument -Tlink.x tells the linker to use link.x as the linker
+#   script. This is usually provided by the cortex-m-rt crate, and by default
+#   the version in that crate will include a file called `memory.x` which
+#   describes the particular memory layout for your specific chip. 
+# * inline-threshold=5 makes the compiler more aggressive and inlining functions
+# * no-vectorize-loops turns off the loop vectorizer (seeing as the M0+ doesn't
+#   have SIMD)
 rustflags = [
     "-C", "link-arg=--nmagic",
     "-C", "link-arg=-Tlink.x",
     "-C", "inline-threshold=5",
     "-C", "no-vectorize-loops",
 ]
+
+# This runner will make a UF2 file and then copy it to a mounted RP2040 in USB
+# Bootloader mode:
 runner = "elf2uf2-rs -d"
+
+# This runner will find a supported SWD debug probe and flash your RP2040 over
+# SWD:
 # runner = "probe-run-rp --chip RP2040"