Link to Cosmos DB React article

[cfleschhut]

Purpose

• Adds link to accompanying article/video tutorial on Azure docs site to README
  For people initially landing on this repo and wondering where to find the videos (like I just did) 🤓
• Removes leftover code fragment in /src/App.js

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

npm t

What to Check

Verify that the following are valid

• Added correct URL to article
• <App /> component w/ <app-heroes /> removed renders correctly

Other Information

[msftclas]

All CLA requirements met.

[bbarrows89]

@microsoftopensource not sure who to bug, but this is a good PR!