Document how to build binary from sources

[iBobik]

I readme there is link to wiki, but there is no such info.

Will be nice if that allow us to build it for all platforms supported by Micropython.

[iBobik]

Build instructions are in this PDF:
https://github.com/letscontrolit/uPyEasy/blob/master/docs/uPyEasy%20Building%20Environment.pdf

It there some good reason why not to write docs in markdown and link it from README?

[iBobik]

Question about docs moved to issue #23, links are in PR.

[iBobik]

Aha, I see that in document Building Environment there are not info about how to build firmware uPyEasy (that binaries offered in README), but how to build MicroPython.