Document nimble build options, esp. -d:release

[snej]

Please document the options/flags the nimble build command takes. There are currently no docs that I can find; the help text just says "[opts, ...]", and the README just says it builds in debug mode.

Justification: Yesterday I tried to build and profile an optimized version of my code, and spent more than an hour trying to figure out why it was so slow — it looked like there was a terrible bottleneck in the Table class. See this forum thread. The answer turned out to be that I didn't have an optimized build at all — I had put -d:release in my nim.cfg file, where it didn't work, instead of putting it on the command line after nimble build. I did this because I knew compiler flags could be put in the config file, and I had no idea what flags could be used with nimble build.

I know nimble build is used during development, and normally that implies a debug configuration, but it's not unusual to do profiling, or check binary sizes, during development too.

[SolitudeSF]

nimble build just forwards arguments to compiler. read nim --fullhelp

[ghost]

That's not a Nimble bug - see nim-lang/Nim#14272

[snej]

nimble build just forwards arguments to compiler.

Great. Then document that in the help text and the README, please.

[dom96]

Then document that in the help text and the README, please.

PRs for this should be easy, please? :)

[genotrance]

This is now covered in the nimble docs.