Adding requirement for a Travis account

CONTRIBUTING.md

@@ -4,6 +4,7 @@
 * [Sign your commits](https://rchain.atlassian.net/wiki/spaces/DOC/pages/498630673/How+to+sign+commits+to+rchain+rchain)
 * [Code Review Process](https://rchain.atlassian.net/wiki/spaces/DOC/pages/44040200/Code+Review+Process)
 * [Github Workflow](https://rchain.atlassian.net/wiki/spaces/DOC/pages/44007462/Github+Fork-n-Beans+Workflow)
+* We use [Travis CI](https://travis-ci.org) for unit testing. We require passage of unit tests to merge PRs. To start this automated testing in a PR, please assure you have an account with Travis based on your GitHub account. See [Travis CI](https://travis-ci.org) to confirm or create your account.
 * Information about setting up your development environment and the structure of the RChain GitHub repository is available in [RChain/README.md](https://github.com/rchain/rchain/blob/master/README.md).
 
 If you're picking up someone else's pull request, then whatever you