Add the showcase repo to Readme.md to give it more exposure to our developers (#8658)

# 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](../CONTRIBUTING.md) 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
> ```bash
> 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](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

README.md

@@ -175,7 +175,8 @@ These binaries interact with nu via a simple JSON-RPC protocol where the command
 If the plugin is a filter, data streams to it one element at a time, and it can stream data back in return via stdin/stdout.
 If the plugin is a sink, it is given the full vector of final data and is given free reign over stdin/stdout to use as it pleases.
 
-The [awesome-nu repo](https://github.com/nushell/awesome-nu#plugins) lists a variety of nu-plugins.
+The [awesome-nu repo](https://github.com/nushell/awesome-nu#plugins) lists a variety of nu-plugins while the [showcase repo](https://github.com/nushell/showcase) *shows* off informative blog posts that have been written about Nushell along with videos that highlight technical
+topics that been presented.
 
 ## Goals