README.build.md add mbed3 build info

Signed-off-by: Andy Green <andy.green@linaro.org>

README.build.md

@@ -216,6 +216,49 @@ Then to set an option and build (for example turn off SSL support):
 or
 	cmake -DLWS_WITH_SSL:BOOL=OFF ..
 
+Building on mbed3
+-----------------
+MBED3 is a non-posix embedded OS targeted on Cortex M class chips.
+
+https://www.mbed.com/
+
+It's quite unlike any other Posixy platform since the OS is linked statically
+in with lws to form one binary.
+
+At the minute server-only is supported and due to bugs in mbed3 network support,
+the port is of alpha quality.  However it can serve the test html, favicon.ico
+and logo png and may be able to make ws connections.  The binary for that
+including the OS, test app, lws and all the assets is only 117KB.
+
+0) Today mbed3 only properly works on FRDM K64F $35 Freescale Dev Board with
+1MB Flash, 256KB SRAM and Ethernet.
+
+http://www.freescale.com/products/arm-processors/kinetis-cortex-m/k-series/k6x-ethernet-mcus/freescale-freedom-development-platform-for-kinetis-k64-k63-and-k24-mcus:FRDM-K64F
+
+1) Get a working mbed3 environment with arm-none-eabi-cs toolchain
+(available in Fedora, Ubuntu and other distros)
+
+2) Confirm you can build things using yotta by following the getting started guide here
+
+https://docs.mbed.com/docs/getting-started-mbed-os/en/latest/
+
+3)
+
+git clone https://github.com/warmcat/lws-test-server
+
+and cd into it
+
+4) mkdir -p yotta_modules ; cd yotta_modules
+
+5) git clone https://github.com/warmcat/libwebsockets ; mv libwebsockets websockets ; cd ..
+
+6) yotta target frdm-k64f-gcc
+
+7) yotta install
+
+8) yotta build
+
+
 Unix GUI
 --------
 If you have a curses-enabled build you simply type: