Add installation instructions to README.
Add installation instructions to README.

klousiaj/passport-oauth2,caco0516/passport-oauth2-libre,tsabolov/passport-oauth2,jaredhanson/passport-oauth2,tartavull/passport-oauth2,nfroidure/passport-oauth2,openknowl/passport-openknowl,theogravity/passport-oauth2-alt,matuszeman/passport-oauth2,pristineio/passport-salesforce,gastonelhordoy/passport-oauth2,aitnasser/passport-oauth2