README: Unix: Describe pycopy-dev variant.

Change-Id: I413b6cdd5186e443f9e7478f8894439e83323cb5
Signed-off-by: Paul Sokolovsky <pfalcon@users.sourceforge.net>

README.md

@@ -163,6 +163,14 @@ Browse available modules on [PyPI](https://pypi.org/search/?q=pycopy-).
 Standard library modules come from
 [pycopy-lib](https://github.com/pfalcon/pycopy-lib) project.
 
+`pycopy` executable built following the instructions above is a
+"production" executable for native Pycopy software. It's also possible
+to build `pycop-dev` executable which provides additional reflection,
+diagnostics, and extensibility capabilities, at the expense of code
+size and memory usage efficiency. In particular, `pycopy-dev` is more
+compatible with software written for CPython. To build the `pycopy-dev`
+variant, run `make dev`.
+
 External dependencies
 ---------------------