Readme.md updates

[K3es]

This is my first contribution to the Septimus project.

I noticed some typos and irregularities when reading the readme.md. So, I altered some of them and hopefully made it a little more uniform.

• Changed all mentions of the word "modlist" or "modlists" to "Modlist" and "Modlists" to mark them as important or significant. This was already done for some.
• Changed all mentions of the Discord server to be more uniform and regular. Adding two more mentions of it in places where it felt natural.
• Added the Issues page to the top of the page to make sure the community knows that Septimus also offers support through the Issues page. This can be seen at line 19.

[jaaimino]

Hey sorry. I need to go back over and see what we want to do with this. I'm in the middle of re-doing some of the docs. Feel free to reach out on discord soon.