feat: Add reviewer checklist

docs/ReviewChecklist.md

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+# Review checklist
+
+This is a review checklist for Lean content going into ./Physlib or ./QuantumInfo.
+It does not cover meta-programs, only standard additions of definitions and lemmas to Lean.
+
+This should not be taken as a definitive guide, as every PR will be different.
+
+There is some subjectivity in reviewing PRs, so instead of framing this in terms of hard-set rules,
+this review check list is framed in terms of questions a reviewer might want to ask themsevles
+when reviewing a PR.
+
+## Level 1: Subject area
+
+- Is the content added physics?
+- Is the content added mainstream physics?
+
+## Level 2: Organisation
+
+- For every definition and lemma added in the PR, could I determine from the file-structure only
+  where I would find that result?
+- Does every definition and lemma added in the PR fit the theme of the rest of the file, or
+  does it fit more appropriately elsewhere?
+- If there are new files made, can I guess the content of that file from the name?
+- Within a given file, can I understand the relevance of results which sit next to each other,
+  or in the same section?
+
+## Level 3: Content
+
+- Is a definition or lemma a simple rewrite of a result already in Mathlib or Physlib?
+- Do the definitions added look generally useful, do they appear in physics, or
+  do they allow you to ask interesting questions about a data-structure that exists?
+- Are lemma statements and definition terms easy to read?
+- Do lemmas seem like they will be useful, and are written in a reusable way?
+- Are the hypotheses of the lemma minimal and necessary?
+- Do any of the have statements inside the proof contain general mathematical identities or
+  physical context that should be extracted into their own standalone lemmas?
+
+## Level 4: Naming and documentation
+
+- Are the names written in Mathlib/Physlib convention?
+- From the name of a declaration can you guess what the declaration is without looking at it?
+- Are the namespaces correct and make sense?
+- Does the module documentation actually help either with the flow of the document or one
+  understand what is in the file, or is it just padding?
+- Does documentation on declarations help one understand what that declaration does?
+
+## Level 5:
+
+- Does the code look neat and clean, or is it difficult to read?
+- Are there any newlines in proofs?