zephyr/README: Update required Zephyr version and mention new features.

Signed-off-by: Damien George <damien@micropython.org>

ports/zephyr/README.md

 MicroPython port to Zephyr RTOS
 ===============================
 
-This is an work-in-progress port of MicroPython to Zephyr RTOS
+This is a work-in-progress port of MicroPython to Zephyr RTOS
 (http://zephyrproject.org).
 
-This port requires Zephyr version 1.8 or higher. All boards supported
+This port requires Zephyr version 2.3.0, and may also work on higher
+versions.  All boards supported
 by Zephyr (with standard level of features support, like UART console)
 should work with MicroPython (but not all were tested).
 
@@ -12,12 +13,14 @@ Features supported at this time:
 
 * REPL (interactive prompt) over Zephyr UART console.
 * `utime` module for time measurements and delays.
-* `machine.Pin` class for GPIO control.
+* `machine.Pin` class for GPIO control, with IRQ support.
 * `machine.I2C` class for I2C control.
 * `usocket` module for networking (IPv4/IPv6).
 * "Frozen modules" support to allow to bundle Python modules together
   with firmware. Including complete applications, including with
   run-on-boot capability.
+* virtual filesystem with FAT and littlefs formats, backed by either
+  DiskAccess or FlashArea (flash map).
 
 Over time, bindings for various Zephyr subsystems may be added.
 
@@ -28,17 +31,35 @@ Building
 Follow to Zephyr web site for Getting Started instruction of installing
 Zephyr SDK, getting Zephyr source code, and setting up development
 environment. (Direct link:
-https://www.zephyrproject.org/doc/getting_started/getting_started.html).
+https://docs.zephyrproject.org/latest/getting_started/index.html).
 You may want to build Zephyr's own sample applications to make sure your
 setup is correct.
 
-To build MicroPython port, in the port subdirectory (zephyr/), run:
+If you already have Zephyr installed but are having issues building the
+MicroPython port then try installing the correct version of Zephyr via:
 
-    make BOARD=<board>
+    $ west init zephyrproject -m https://github.com/zephyrproject-rtos/zephyr --mr v2.3.0
+
+Alternatively, you don't have to redo the Zephyr installation to just
+switch from master to a tagged release, you can instead do:
+
+    $ cd zephyrproject/zephyr
+    $ git checkout v2.3.0
+    $ west update
+
+With Zephyr installed you may then need to configure your environment,
+for example by sourcing `zephyrproject/zephyr/zephyr-env.sh`.
+
+Once Zephyr is ready to use you can build the MicroPython port.
+In the port subdirectory `ports/zephyr/` run:
+
+    $ make BOARD=<board>
 
 If you don't specify BOARD, the default is `qemu_x86` (x86 target running
-in QEMU emulator). Consult Zephyr documentation above for the list of
-supported boards.
+in QEMU emulator).  Consult the Zephyr documentation above for the list of
+supported boards.  Board configuration files appearing in `ports/zephyr/boards/`
+correspond to boards that have been tested with MicroPython and may have
+additional options enabled, like filesystem support.
 
 
 Running
@@ -66,6 +87,10 @@ cf. for example QEMU networking requirements above; real hardware boards
 generally should not have any special requirements, unless there're known
 issues).
 
+For example, to deploy firmware on the FRDM-K64F board run:
+
+    $ make BOARD=frdm_k64f flash
+
 
 Quick example
 -------------
@@ -89,6 +114,19 @@ reference materials). To execute the above sample, copy it to clipboard, in
 MicroPython REPL enter "paste mode" using Ctrl+E, paste clipboard, press
 Ctrl+D to finish paste mode and start execution.
 
+To respond to Pin change IRQs, on a FRDM-K64F board run:
+
+    from machine import Pin
+
+    SW2 = Pin(("GPIO_2", 6), Pin.IN)
+    SW3 = Pin(("GPIO_0", 4), Pin.IN)
+
+    SW2.irq(lambda t: print("SW2 changed"))
+    SW3.irq(lambda t: print("SW3 changed"))
+
+    while True:
+        pass
+
 Example of using I2C to scan for I2C slaves:
 
     from machine import I2C