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>

README.md

@@ -2,7 +2,7 @@
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/bwmarrin/discordgo.svg)](https://pkg.go.dev/github.com/bwmarrin/discordgo) [![Go Report Card](https://goreportcard.com/badge/github.com/bwmarrin/discordgo)](https://goreportcard.com/report/github.com/bwmarrin/discordgo) [![Build Status](https://travis-ci.com/bwmarrin/discordgo.svg?branch=master)](https://travis-ci.com/bwmarrin/discordgo) [![Discord Gophers](https://img.shields.io/badge/Discord%20Gophers-%23discordgo-blue.svg)](https://discord.gg/golang) [![Discord API](https://img.shields.io/badge/Discord%20API-%23go_discordgo-blue.svg)](https://discord.com/invite/discord-api)
 
-<img align="right" src="https://github.com/bwmarrin/discordgo/blob/master/docs/img/discordgo.png">
+<img align="right" alt="DiscordGo logo" src="docs/img/discordgo.svg" width="400">
 
 DiscordGo is a [Go](https://golang.org/) package that provides low level 
 bindings to the [Discord](https://discord.com/) chat client API. DiscordGo