Add some information to help new people use the skeleton

[thePalindrome]

So, firstly, in the gradle.properties, where it says "don't put it here", it'd be nice to note that you can put a gradle.properties in your GRADLE_USER_HOME (default on linux is ~/.gradle, probably the same on osx, and likely %APPDATA%/gradle or %HOME%/.gradle on windows).

you can just set mc_username, which will keep the name static but won't be valid for anything else (including skins iirc). Specifying uuid will grant you the skin, although it may still have issues with mods that will use your uuid (ftblib and thaumcraft both do).

If you set mc_password, it will attempt to log in with it in combination with your mc_username (which for logging in must be your email address (your username for the mojang account), rather than your MC name). Also note that if mc_username, mc_password, and mc_uuid are all set, you will be logged in (and the console log will say as such), but you will not be given a session!

In regards to editing the build.gradle, it would be amazing to put in some docs about what are "user-servicable". Right now, that is jar metadata (being hardcoded in the build.gradle), and repositories/dependencies. Ideally those should be moved to the gradle.properties, as it can very quickly overwhelm anyone not immediately familiar with the gradle system.

Some brief documentation about "secret.json" or whatever it is set to (I'm noting you can specify your own file with an envvar). Just the basics of "this is for maven, curseforge, and a few other services." should be enough, as at that point it is then clear what documentation needs to be looked up.