Update the PR merge criteria to allow situational judgement

[reyang]

There was a discussion among TC members about whether we should require 4 approvals for OTEPs.

Given we want to merge OTEPs into the specification repo, I think it'll be good to improve the spec PR merge criteria. Here is the thinking behind this change:

1. Having 2 approvals seem good to me, I haven't seen cases where people try to abuse it.
2. Increasing the number from 2 to 4 will slow down PRs for trivial/editorial changes. Many of the editorial PRs were contributed by first-time contributor to the OpenTelemetry project. I actually think we might want to speed this up rather than slowing it down.
3. Since the project started, I think the spec maintainers (TC members) have been careful about significant changes, so we make sure people have enough time to review, discuss and share their opinions.

Changes

Clarify that spec maintainers might put additional requirements before merging the PR.

For non-trivial changes, follow the change proposal process.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[arminru]

Follow-up: Once open-telemetry/oteps#266 is approved and implemented, I think this list should explicitly mention the 4 approvals required for new OTEPs (and less for editorial fixes on merged OTEPs, if necessary) rather than this being a situational judgement call.

[carlosalberto]

cc @jsuereth