docs: fix link to the design docs in README.md

[bobtfish]

Fix link in README.md

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• [*] I acknowledge that all my contributions will be made under the project's license.

[welcome]

Congratulations on your first contribution to the Serverless Toolkit!
If you are on the look out for more ways to contribute to open-source, check out a list of some of our repositories at https://github.com/twilio/opensource.
If you want to stay up-to-date with Twilio's OSS activities, subscribe here: https://twil.io/oss-updates
And if you love Twilio as much as we do, make sure to check out our Twilio Champions program!