Add PR template for new SPECs (#393)

Author: drammock

.github/PULL_REQUEST_TEMPLATE/new_spec.md

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+<!-- Thanks for proposing a new SPEC! Make sure your PR title is descriptive and short. -->
+
+## Attestation
+
+- [ ] I have read and understood the [SPEC Purpose and Process](https://scientific-python.org/specs/purpose-and-process/) guidelines.
+
+## Description
+
+<!--- Please summarize the purpose of this new SPEC, in 1-3 sentences. -->
+
+## Link to SPEC Committee's "viability" discussion thread (required)
+
+<!--
+Insert a link to this proposal's initial viability discussion thread in
+https://discuss.scientific-python.org/c/specs/ideas
+-->
+
+## Description of (or link to) exploratory discussions/POCs (optional)
+
+<!--
+SPEC guidelines suggest a first step of one of the following:
+
+1. discuss the idea with at least one project in the ecosystem,
+1. discuss the idea with at least one other member of the ecosystem, or
+1. create a minimal, proof of concept (POC) prototype.
+
+(see https://scientific-python.org/specs/purpose-and-process/#new-spec-proposals).
+In this field, include a description of those preliminary steps and/or links to public discussions or POCs.
+-->

.github/pull_request_template.md

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+If you are opening this PR to propose a new SPEC, please go to the `Preview` tab,
+then [click here for the new SPEC template](?expand=1&template=new_spec.md).
+Otherwise, delete this text and proceed with your PR as normal.