docs: Initial split of README into docs directory

The top-level README document has got pretty unwieldy and so we need to
switch to add some more structure to our documentation. The first step
is to split out sub-sections of the README document into separate files
in a new 'docs' directory.

Whilst splitting up the README, we can also take the opportunity to tidy
things up and fix a couple of typos.

Signed-off-by: Paul Barker <pbarker@toganlabs.com>

docs/layer-contents.md

@@ -0,0 +1,24 @@
+# Layer Contents
+
+## Supported Machines
+
+* raspberrypi
+* raspberrypi0
+* raspberrypi0-wifi
+* raspberrypi2
+* raspberrypi3
+* raspberrypi3-64 (64 bit kernel & userspace)
+* raspberrypi-cm (dummy alias for raspberrypi)
+* raspberrypi-cm3 (dummy alias for raspberrypi2)
+
+## Images
+
+* rpi-hwup-image
+  * Hardware up image
+
+* rpi-basic-image
+  * Based on rpi-hwup-image with some added features (ex: splash)
+
+* rpi-test-image
+  * Image based on rpi-basic-image which includes most of the packages in this
+    layer and some media samples.