Update documentation to explicitly ignore errors returned by `AddVertex`

[dominikbraun]

At the moment, AddVertex calls in the documentation (README + GoDoc) look like this:

g.AddVertex(1, 2)

But AddVertex returns an error, and that should be made clear in the docs. The error should be ignored explicitly, just as with the AddEdge calls:

_ = g.AddVertex(1, 2)

Again, this should be done in both the README and the code comments.

[vinicio]

Hi! Can I take this one?

[dominikbraun]

Sure 🙂