Replace todo files with README.md

[ashleywillard]

• I bumped the gem version (or don't need to) 💎

Current behavior

When creating a new pack with packs, you get README_TODO.md and app/public/TODO.md files by default. In contrast, pks creates only a README.md file.

Desired behavior

packs and pks should create the same README files. This PR updates packs to behave like pks. I chose to change packs because:

1. The README_TODO.md and app/public/TODO.md files are often not replaced.
2. Documentation of the public API typically lives in the README.md file, so we don't really need a separate file telling you to add to README.md.

Limitations

Note that this change brings packs more in line with pks, but pks still creates a few more folders that packs does not (e.g. spec).