Compiler selection doco (#6115)
Compiler selection doco (#6115)

* Slices indices explanation

as per https://forum.nim-lang.org/t/2870

* Remove duplicate in note

* Doco: Selecting a different compiler

tulayang/Nim,tulayang/Nim,tulayang/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,russpowers/Nim,dom96/Nim,russpowers/Nim,russpowers/Nim,tulayang/Nim,dom96/Nim,tulayang/Nim,tulayang/Nim,russpowers/Nim,dom96/Nim,dom96/Nim,russpowers/Nim,dom96/Nim,dom96/Nim