chore: update README.md (#162)

## Description

Adds package description to readme.

## Related Issue

Fixes #
<!-- or -->
Relates to #

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [X] Other (security config, docs update, etc)

## Checklist before merging

- [X] Test, docs, adr added or updated as needed
- [X] [Contributor Guide
Steps](https://github.com/defenseunicorns/uds-package-docuseal/blob/main/CONTRIBUTING.md#developer-workflow)
followed

README.md

@@ -7,6 +7,8 @@
 
 This package is designed for use as part of a [UDS Software Factory](https://github.com/defenseunicorns/uds-software-factory) bundle deployed on [UDS Core](https://github.com/defenseunicorns/uds-core).
 
+> Mattermost is a secure collaboration platform that accelerates mission-critical work by offering persistent chat, workflow automation, and integrated voice, screen, file, and content sharing. It is built for organizations with rigorous and complex environments, such as government, defense, and critical infrastructure.
+
 > [!IMPORTANT]  
 > The `arm64` package includes `amd64` images due to lack of availability of `arm64` images from upstream projects at this time. This means you can deploy the `arm64` package on an `arm64` kubernetes cluster, but some of the images contained in the package will require emulation (e.g., qemu or rosetta) to run properly.