Vectorization of DiscordGo mark (#1099)

* Add vector imagination of discordgo.png

* Delete discordgo.png

* Update README.md to use vector artwork

Added alternative text and used relative reference to artwork

* README.md more meaningful alt text

Co-authored-by: Fedor Lapshin <fe.lap.prog@gmail.com>

Co-authored-by: Fedor Lapshin <fe.lap.prog@gmail.com>