Make evaluation/intro more user-friendly

[nikpag]

This PR makes the evaluation/intro first-time experience a little more straightforward by:

• Adding a README that prompts the user to create required input files beforehand, in case they decide to run the evaluation scripts (such as demo-spell.sh) separately, as opposed to running test.sh directly.
• Fixing a problem with curl that caused the 100M.txt file to be empty when the data was fetched from www.gutenberg.org.

[github-actions]

OS:ubuntu-20.04
Sun May 28 16:38:35 UTC 2023
intro: 2/2 tests passed.
interface: 39/39 tests passed.
compiler: 54/54 tests passed.
agg: 109/109 tests passed.

[github-actions]

OS:ubuntu-20.04
Tue May 30 19:58:01 UTC 2023
intro: 2/2 tests passed.
interface: 39/39 tests passed.
compiler: 54/54 tests passed.
agg: 109/109 tests passed.

[angelhof]

Thanks for the PR :) It looks great, the issue with the POSIX test has to do with the fact that you did the changes in your own fork (our current CI doesn't support that). Since the change is not pervasive I will just merge!