docs: add River to the list of companies using t3 stack

[bernii]

Hi! I'm Co-founder and CTO of River. We'd like to share some ❤️ since we're having a great experience using T3 stack for our web app :)

✅ Checklist

• I have followed every step in the contributing guide (updated 2022-10-06).
• The PR title follows the convention we established conventional-commit
• I performed a functional test on my final commit

---

Changelog

Add River as an example of company that's using T3 stack.

---

Screenshots

💯

[changeset-bot]

⚠️ No Changeset found

Latest commit: 33c71ea

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[vercel]

@bernii is attempting to deploy a commit to the t3-oss Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
create-t3-app	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 4, 2023 4:53pm

[bernii]

I fixed the PR title since it failed the CI step ^ LMK if you need anything else :)

[juliusmarminge]

Awesome!!