demonstrate use of 'fields' parameter for python client

[andrewsu]

In the documentation at https://pypi.org/project/mygene/, we should add some documentation of the fields parameter. e.g.:

mg.querymany([1017, '695', 'NA_TEST'], scopes='entrezgene', species='human',fields='entrezgene,ensembl,uniprot')

[newgene]

updated in https://github.com/biothings/mygene.py/commit/069b83a2c5a49adce778eaefbf69089549b2afd3

Actually pypi page will be updated upon the next release.