Readme issues

[cfrontin]

Some odds and ends for the README.

1. in this line

   conda CONDA_SUBDIR=osx-arm64 conda create -n ard-env 

   the prepended conda is a typo
2. here, we randomly use mamba rather than the more general conda: mamba install wisdem -y
3. we now have recommendation of "native" install, which should be default on M1 machines unless the user has taken the advanced step of installing an x86 version of conda themselves... by default and with standard/recommended conda installation the extra SUBDIR steps shouldn't be necessary