document minimum supported MicroPython version

Also, what versions and devices everything was tested on.

Everything was tested all the way back to v1.10 today, and the
current code works correctly. However, MicroPython v1.11 has
a bug related to loading the ULP binary, so while py-esp32-ulp
produces correct binaries on that version, they can not actually
be used on the same version. To avoid explaining all this detail,
MicroPython v1.12 was chosen as the minimum supported version.

(Actually MicroPython v1.12 massively reduced its own memory
consumption, which makes it much better for py-esp32-ulp anyway,
especially when assembling larger sources.)

Fixes #48.

Author: wnienhaus

README.rst

@@ -29,6 +29,11 @@ ULP source files containing convenience macros such as WRITE_RTC_REG. The
 preprocessor and how to use it is documented here:
 `Preprocessor support <docs/preprocess.rst>`_.
 
+The minimum supported version of MicroPython is v1.12. py-esp32-ulp has been
+tested with MicroPython v1.12 and v1.17. It has been tested on real ESP32's
+of the type ESP32D0WDQ6 (revision 1) without SPIRAM. It has also been tested
+on the Unix port.
+
 There might be some stuff missing, some bugs and other symptoms of alpha
 software. Also, error and exception handling is rather rough yet.