docs: Introduce spec overview and template (#14839)

docs/specs/README.md

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+# Telegraf Specification Overview
+
+## Objective
+
+Define and layout the Telegraf specification process.
+
+## Overview
+
+The general goal of a spec is to detail the work that needs to get accomplished
+for a new feature. A developer should be able to pick up a spec and have a
+decent understanding of the objective, the steps required, and most of the
+general design decisions.
+
+The specs can then live in the Telegraf repository to share and involve the
+community in the process of planning larger changes or new features. The specs
+also serve as a public historical record for changes.
+
+## Process
+
+The general workflow is for a user to put up a PR with a spec outlining the
+task, have any discussion in the PR, reach consensus, and ultimately commit
+the finished spec to the repo.
+
+While researching a new feature may involve an investment of time, writing the
+spec should be relatively quick. It should not take hours of time.
+
+## Spec naming
+
+Please name the actual file prefixed with `tsd` and the next available
+number, for example:
+
+* tsd-001-agent-write-ahead-log.md
+* tsd-002-inputs-apache-increase-timeout.md
+* tsd-003-serializers-parquet.md
+
+All lower-case and separated by hyphens.
+
+## What belongs in a spec
+
+A spec should involve the creation of a markdown file with at least an objective
+and overview:
+
+* Objective (required) - One sentence headline
+* Overview (required) - Explain the reasoning for the new feature and any
+  historical information. Answer the why this is needed.
+
+Please feel free to make a copy the template.md and start with that.
+
+The user is free to add additional sections or parts in order to express and
+convey a new feature. For example this might include:
+
+* Keywords - Help identify what the spec is about
+* Is/Is-not - Explicitly state what this change includes and does not include
+* Prior Art - Point at existing or previous PRs, issues, or other works that
+  demonstrate the feature or need for it.
+* Open Questions - Section with open questions that can get captured in
+  updates to the PR
+
+## Changing existing specs
+
+Small changes which are non-substantive, like grammar or formatting are gladly
+accepted.
+
+After a feature is complete it may make sense to come back and update a spec
+based on the final result.
+
+Other changes that make substantive changes are entirely up to the maintainers
+whether the edits to an existing RFC will be accepted. In general, finished
+specs should be considered complete and done, however, priorities, details, or
+other situations may evolve over time and as such introduce the need to make
+updates.

docs/specs/template.md

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+# Title
+
+## Objective
+
+One sentence explanation of the feature.
+
+## Overview
+
+Background and details about the feature.
+
+## Keywords
+
+A few items to specify what areas of Telegraf this spec affects (e.g. outputs,
+inputs, processors, aggregators, agent, packaging, etc.)
+
+## Is/Is-not
+
+## Prior art
+
+## Open questions