Add optional depedency in readme
Add optional depedency in readme

sklam/numba,numba/numba,IntelLabs/numba,stuartarchibald/numba,stonebig/numba,gmarkall/numba,IntelLabs/numba,sklam/numba,stuartarchibald/numba,seibert/numba,stuartarchibald/numba,IntelLabs/numba,stuartarchibald/numba,sklam/numba,cpcloud/numba,sklam/numba,numba/numba,cpcloud/numba,numba/numba,cpcloud/numba,stonebig/numba,IntelLabs/numba,gmarkall/numba,numba/numba,seibert/numba,gmarkall/numba,stonebig/numba,gmarkall/numba,cpcloud/numba,IntelLabs/numba,seibert/numba,seibert/numba,cpcloud/numba,stonebig/numba,numba/numba,gmarkall/numba,stonebig/numba,stuartarchibald/numba,sklam/numba,seibert/numba