Clarify that resources are immutable (#515)
Clarify that resources are immutable (#515)

* Clarify that resources are immutable

* Typo.

* Update specification/sdk-resource.md

Co-Authored-By: Armin Ruech <5375d2e98a78036adfbf90c7f7841c3894b65d4b@gmail.com>

* Mention immutability right in the beginning.

* Reformulate Merge to make immutability more clear

Co-authored-by: Armin Ruech <5375d2e98a78036adfbf90c7f7841c3894b65d4b@gmail.com>
Co-authored-by: Bogdan Drutu <3f8e17047c2434e5c82a61403e56b9ef3e226d99@gmail.com>
open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification