Updated Nexus Proxy/Hosted Setup

[dbeezt]

Added documentation for repo. setup based off of official Sonatype docs e.g.: https://help.sonatype.com/repomanager3/nexus-repository-administration/formats/pypi-repositories

This pull request makes the following changes:

• updates general documentation

It relates to the following issue #s:

• documents solution in https://github.com/sonatype-nexus-community/nexus-repository-cargo/issues/11

[sonatypecla[bot]]

Thanks for the contribution! Before we can merge this, we need @dbeezt to sign the Sonatype Contributor License Agreement.