Provide updated readme

[ajmas]

It would be good to have the readme updated to include build instructions and usage instructions.

[ajmas]

I have just started updating the readme and was able to build the project using Maven, but had to make a small change in EventListener.java, changing line 64 to be:

Sender.shutdown(event.getPlugin().getServer(), config.getUrl());

I have now placed the DiscordWebhook-Spigot-1.0.1-jar-with-dependencies.jar jar in the plugins folder and launched Spigot, for MC 1.15.2

Does the current version in GitHub support configuration from the MC command prompt? I don't see anything in CommandHandler.java for this?

[ajmas]

Never mind I see there is config file that gets generated. I will update the docs and see if you are interested in a PR.