README.md: add PIPmiR reference
README.md: add PIPmiR reference

mwolfien/galaxy-rna-workbench,bgruening/galaxy-rna-workbench,mwolfien/galaxy-rna-workbench,bgruening/galaxy-rna-workbench,bgruening/galaxy-rna-workbench,mwolfien/galaxy-rna-workbench