Add contributor license agreement. (#1556)

This PR adds a contributor license agreement (CLA) to the project.
Contributors will need to sign the CLA before we merge their PRs. This
will be handled automatically via a bot here on GitHub.

## Context

Did you know tldraw is a [venture
backed](https://tldraw.substack.com/p/tiny-little-seed-round) company?
Our CLA is designed to protect our options for any future re-licensing
of the project's code or transferring ownership, which can become
complicated when taking public contributions community, but which can
become necessary by fate and/or go to market strategies.

A CLA adds a bit of friction and hassle for contributors, however I'd
prefer to be upfront about our relationship rather than doing what many
open source projects have done in the past, which is relicensing code
contributed without such prior agreement.

If you have any questions, feel free to post them below.

### Change Type

- [x] `internal` — Any other changes that don't affect the published
package (will not publish a new version)

apps/docs/content/community/contributing.mdx

@@ -6,4 +6,10 @@ date: 3/22/2023
 order: 0
 ---
 
-Coming soon.
+Interested in contributing to the open source project? 
+
+You can find tldraw on GitHub at [github.com/tldraw/tldraw](https://github.com/tldraw/tldraw). You can [create an issue](https://github.com/tldraw/tldraw/issues/new/choose) and submit pull requests for our review.
+
+Please see our [Contributing guide](https://github.com/tldraw/tldraw/blob/main/CONTRIBUTING.md) for more information.
+
+Please also see our [Code of Conduct](https://github.com/tldraw/tldraw/blob/main/CODE_OF_CONDUCT.md) for our expectations around contributor culture.