[FR] add supported nim version, compiler options to readme
It would be nice if the nim version owlkettle is being tested with, as well as compiler options used, were documented in the readme. I just tried compiling with nim-1.6.12 and orc and i'm getting errors like:

```
/home/itwrx/.cache/nim/muhapp_d/@mviews@sroutine_list.nim.c:3049:17: error: ‘colontmpD_’ undeclared (first use in this function)
 3049 |                 colontmpD_ = (*((void**) (stateX60gensym3_)));
```
and:

```
/home/itwrx/.cache/nim/muhapp_d/@m..@s..@s..@s..@s..@s.nimble@spkgs@sowlkettle-@hhead@sowlkettle@swidgets.nim.c:26721:17: error: ‘colontmpD_’ undeclared (first use in this function)
26721 |                 colontmpD_ = (*((void**) (stateX60gensym1_)));
```
compiling with refc, boehm, markAndSweep, and go works.

thanks