Fix relative links in README

In theory, github markdown docs say that:

> Links starting with / will be relative to the repository root.

Yet, they are resolved to be relative to the github.com domain. Maybe
it's a special case with the `/docs` , since that also references the
actual github docs.

Changed them to explicit relative links.

Author: particleflux

README.md

@@ -147,7 +147,7 @@ Verify is open-sourced software licensed under the [MIT][9] License.
 [4]: https://chaijs.com/
 [5]: https://jasmine.github.io/
 [6]: https://rspec.info/
-[7]: /docs/supported_verifiers.md
-[8]: /docs/supported_expectations.md
-[9]: /LICENSE
-[10]: /UPGRADE.md
+[7]: ./docs/supported_verifiers.md
+[8]: ./docs/supported_expectations.md
+[9]: ./LICENSE
+[10]: ./UPGRADE.md