Update nordic board documentation [was: Rename nrf52dk_base to nrf52_base]

boards/nordic/README.md

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+Nordic Family of Boards
+=======================
+
+There are several closely related boards from Nordic supported, or previously
+supported, by Tock. Unfortunately, the Nordic naming scheme is a bit confusing,
+so two notions get conflated:
+
+  - The `nrf52xxx` family of chips, specifically for Tock the nrf52832 and nrf52840, is often referred to as the "nrf52" family of chips.
+  - The development kit for the nrf52840 is called the nrf52840dk; the development kit for the nrf52832 is called nrf52dk.
+
+The result is:
+
+  - "nrf52" => nrf52832 chip or nrf52840 chip, which are on the nrf52dk and nrf52840dk respectively
+  - "nrf52dk" => development board with the nrf52832 chip
+  - "nrf52840dk" => development board with the nrf52840 chip
+  - "nrf52840_dongle" => minimalist board with the nrf52840 chip
+
+This naming matches the products released by Nordic, but users should be careful.
+
+Additionally, the acd52832 board is a platform developed by Aconno that
+features a nrf52832 chip.
+
+
+Tock Hierarchy
+--------------
+
+As much is shared across these platforms, Tock uses an "nrf52dk_base" crate.
+This is a library crate that contains code shared by boards that include any
+chips from the nrf52 family.
+
+
+Legacy Boards
+-------------
+
+Tock 1.3 was the last release with support for the nrf51, a old chip in this
+family that has a Cortex M0 with no MPU.
+
+Code for the nrf51dk platform is available in the
+[tock archive](https://github.com/tock/tock-archive/tree/master/nrf51dk).