Add some information about GHDL backend issues

A number of people have tripped up on GHDL backend issues. We
require either the LLVM or gcc backend, because the mcode backend
can't link against libraries which we use for simulated memory
and UART. Make that clearer, and point people at the Docker images
if they are having issues building ghdl.

Also point at the prebuilt micropython image.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

README.md

@@ -26,8 +26,17 @@ make -j$(nproc)
 cd ../../../
 ```
 
+  A prebuilt micropython image is also available in the micropython/ directory.
+
 - Microwatt uses ghdl for simulation. Either install this from your
-  distro or build it. Next build microwatt:
+  distro or build it. Microwatt requires ghdl to be built with the LLVM
+  or gcc backend, which not all distros do (Fedora does, Debian/Ubuntu
+  appears not to). ghdl with the LLVM backend is likely easier to build.
+
+  If building ghdl from scratch is too much for you, the microwatt Makefile
+  supports using Docker or podman images. Read through the Makefile for details.
+
+- Next build microwatt:
 
 ```
 git clone https://github.com/antonblanchard/microwatt
@@ -39,6 +48,12 @@ make
 
 ```
 ln -s ../micropython/ports/powerpc/build/firmware.bin main_ram.bin
+```
+
+  Or if you were using the pre-built image:
+
+```
+ln -s micropython/firmware.bin main_ram.bin
 ```
 
 - Now run microwatt, sending debug output to /dev/null: