Document r_clang compiler option

[mfansler]

The r-arrow recipe uses an r_clang compiler on Windows:

https://github.com/conda-forge/r-arrow-feedstock/blob/99eee2298644d6439115a96ca5fd47f23378f353/recipe/meta.yaml#L31

However, the documentation only describes five compilers, which does not include this:

conda-forge.github.io/src/maintainer/knowledge_base.rst

Lines 280 to 287 in 1d3214c

	There are currently five supported compilers:
	- C
	- cxx
	- Fortran
	- Go
	- Rust

This should be documented with guidance on usage.

[xhochy]

More background can be found on this compiler (and its history) on https://uwekorn.com/2020/06/14/r-arrow-for-conda-windows.html

[mfansler]

@xhochy nice! That's a wonderful case study you put together there!