(absolute) link to CONTRIBUTING.md

[butuzov]

link to contributing.md changed to absolute

Reason for this - is website https://serverless-api.twilio-labs.com/ that generated by this readme
doesn't actually have CONTRIBUTING.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.

[dkundel]

Thank you so much for your contribution! We are excited about every contribution and would love to send you some swag if possible. You can read more about it and apply for it here: https://twil.io/hacktoberfest-swag

[butuzov]

Cool! Thanks!