qemu-arm: Add port README.

Based on https://github.com/micropython/micropython/pull/630 by @errordeveloper.

qemu-arm/README.md

@@ -0,0 +1,23 @@
+This is experimental, community-supported port for Cortex-M emulation as
+provided my QEMU (http://qemu.org).
+
+The purposes of this port are to enable:
+
+1. Continuous integration
+    - run tests agains architecture-specific parts of code base
+2. Experimentation
+    - simulation & prototyping of anything that has architecture-specific
+      code
+    - exploring instruction set in terms of optimising some part of
+      MicroPython or a module
+3. Streamlined debugging
+    - no need for JTAG or even an MCU chip itself
+    - no need to use OpenOCD or anything else that might slow down the
+      process in terms of plugging things together, pressing buttons, etc.
+
+This port will only work with with [GCC ARM Embedded](launchpad.net/gcc-arm-embedded)
+toolchain and not with CodeSourcery toolchain. You will need to modify
+`LDFLAGS` if you want to use CodeSourcery's version of `arm-none-eabi`.
+The difference is that CodeSourcery needs `-T generic-m-hosted.ld` while
+ARM's version  requires `--specs=nano.specs --specs=rdimon.specs` to be
+passed to the linker.