README.md: update with information about compilation and installation

[pietvo]

I wanted to compile libcmini myself, but I found no information about the various flags and options. So I figured this out and put the information in README.md for future reference.
It supposes that my other change about STDIO_MAP_NEWLINE has been applied.

[th-otto]

I will look into this, but i think that STDIO_MAP_NEWLINE behaviour should be made the default (because it is more likely that programs compiled with libcmini are used under plain TOS; for MiNT you are better using mintlib).

[pietvo]

I agree that it would be better if STDIO_MAP_NEWLINE behaviour were the default, but I didn't want to make that decision. If that is your decision, I will change it in the Makefile PR, and in the documentation.