Add building, testing, and testnet instructions on README

[vitsalis]

This PR introduces some simple instructions for performing common operations inside the README.md file. Some of those might be too specific for the final version of the README file and can be moved to more specific files in the future.

[aakoshh]

So useful, thanks a lot! 🏅

My only suggestion is to use console or sh highlighting for the code blocks.

I used to use console with $ because Markdown will color the prompt and the command output nicely. It only coloured in the first > out of >>>.

However, somebody pointed out to me that I shouldn't do this because people will try to copy paste the command and get into trouble with the leading $. The recommendation was to keep the output separately. This seemed annoying, but I just found this style guide that recommends the same thing.

Perhaps console with prompts is best used for showing an example session, with multiple commands. But your commands do look like things that somebody would want to copy to their console to run a testnet, so the >>> or $ can become irritating.

Since you aren't showing the output most of the time, the prompt could be removed at least for those commands, and they can use sh or bash. I reckon the ls example can be with the prompt and the output, because it's more concise than including the output in prose.

[aakoshh]

What would it take to move to 1.18?

[vitsalis]

Not sure exactly, but I figure we should stick to 1.17 since v0.45.4 of the SDK uses it

[vitsalis]

I don't see any apparent issues when changing the first line of go.mod to 1.18 and building though.

[vitsalis]

Thanks @aakoshh for the suggestions and explanation! Addressed the comments, please check.

[aakoshh]

🎉