Github: add how to install

[lfpaulin]

currently works with
conda create --name STRdust -c bioconda python3.7 pysam spoa pyspoa mreps
conda activate STRdust

//STRdust .py

[wdecoster]

Eventually, when we feel it is ready, I'll add STRdust to bioconda directly so users can do conda install strdust and then do STRdust <phased-alignment.bam>.