CONTRIBUTING: Make it more explicit you need to sign the CLA (open-telemetry#2582)

Author: xrmx

CONTRIBUTING.md

@@ -13,6 +13,10 @@ on how to become a [**Member**](https://github.com/open-telemetry/community/blob
 [**Approver**](https://github.com/open-telemetry/community/blob/main/community-membership.md#approver)
 and [**Maintainer**](https://github.com/open-telemetry/community/blob/main/community-membership.md#maintainer).
 
+Before you can contribute, you will need to sign the [Contributor License Agreement](https://docs.linuxfoundation.org/lfx/easycla/contributors).
+
+Please also read the [OpenTelemetry Contributor Guide](https://github.com/open-telemetry/community/blob/main/CONTRIBUTING.md).
+
 ## Index
 
 * [Find a Buddy and get Started Quickly](#find-a-buddy-and-get-started-quickly)