Clarify that issues must be created before PRs

The contributing guide says all significant changes should go through OTEP
process. It also describes an issue workflow which says that an issue is
created first, the issue is triaged and accepted and then a PR is created,
however the "Proposing a change" section does not explicitly describe this
as a possibility.

I added a paragraph to make it clear that there are 3 possible options:
with an OTEP, with an issue and trivial changes that are just PRs.

Unless we make this change it appears that everything that is not a trivial
change must go through OTEP, which is not our current process and is not
what we want.

CONTRIBUTING.md

@@ -16,8 +16,19 @@ Agreement](https://identity.linuxfoundation.org/projects/cncf).
 Significant changes should go through the [OpenTelemetry Enhancement
 Proposal](https://github.com/open-telemetry/oteps) process.
 
-Small changes, such as clarifications, wording changes, spelling/grammar
-corrections, etc. can be made directly via pull requests.
+Smaller changes can follow a shorter process:
+
+- [Create an issue](https://github.com/open-telemetry/opentelemetry-specification/issues/new/choose)
+  in this repo and describe the proposed change.
+- Follow the [issue workflow](#issue-triaging) and make sure the issue is accepted with
+  a "Yes" response. If the response to the issue is not "Yes" then do not create a PR
+  that implements the change since it will be rejected.
+- Once the issue is accepted the assignee can work on it and create a PR that implements
+  the change described in the issue.
+
+Trivial changes, such as clarifications, wording changes, spelling/grammar
+corrections, etc. can be made directly via pull requests and do not require an associated
+issue.
 
 ## Writing specs