Add the showcase repo to Readme.md to give it more exposure to our developers

[stormasm]

Description

The showcase repo has some cool material in it that I think is valuable for our readers developers. Giving it a bit more exposure by putting it our Readme will allow more folks to know this is yet another valuable resource. I learned about this today from Darren and thought it would be good to allow others the same opportunity to find it easier as well...

(Thank you for improving Nushell. Please, check our contributing guide and talk to the core team before making major changes.)

(Description of your pull request goes here. Provide examples and/or screenshots if your changes affect the user experience.)

User-Facing Changes

(List of all changes that impact the user experience here. This helps us keep track of breaking changes.)

Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace to check that all tests pass

Note
from nushell you can also use the toolkit as follows

use toolkit.nu  # or use an `env_change` hook to activate it automatically
toolkit check pr

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.