Various Gremlin.Net documentation updates

We have some parts of the documentation regarding Gremlin.Net that should be updated now that we have a stable release:

• Homepage: Query Languages misses a reference to Gremlin.Net
• Homepage: Language Drivers section still links to the repository on my private account. It should probably link to the Gremlin.Net section in the reference docs or to a README (see below).
• Reference docs still contain a warning that Gremlin.Net does not yet have an official release.
• Add a README that explains Gremlin.Net, similar to the documents we have for the Python and Javascript GLVs.
• The csproj file contains a pretty extensive summary element, but summary isn't part of the csproj standard. So it isn't used for anything. It should be merged into the description element instead.