README and variables.tf formatting

Moved all variables in the README to a table format (to hopefully help visibility) and added descriptions to variables that were missing them in variables.tf. Also threw in empty lines between each of the variable declarations to help readability too (even though VS Code helps with spacing anyways).

Would be interesting to see if we could either auto-generate or at least validate that the README has the same content/variables as what's in the actual variables.tf to prevent them from drifting from each other. Might be a fun little student project 😉